I got some time to sit down, so I can take a look for you! I'm more than happy to talk about anything I say here, so if you have any further questions don't hesitate to ask.
Before I begin, I just want to say that what you're experiencing is very normal. It takes time for a site to get off the ground and the early stages are pretty hard. Nothing you're describing sounds unusual, so if you just keep at it I'm sure it'll be fine. That said, I'll still take a look at everything else so your site can be the best it can be.
I see you're using a premade skin, so I won't focus too much on that. What you have looks fine. The colors work well together and I can read your text just fine. The contrast in the text is very good. At most, I'd suggest incorporating some of that nice red in the background as an accent color, but this is not really necessary. I do notice that you don't have a link to the recent threads or recent posts anywhere. Recent posts especially are one of the perks of using Proboards over jcink, so that might be worth adding. You can just add it to your navigation by going to Admin -> Settings -> General Settings -> Navigation and putting
heroreality.boards.net/posts/recent as the URL. I'm having a little bit of trouble reading your usergroup colors. This skin is fairly dark, so something lighter will be a bit easier to read. Maybe something like #31c36f (green), #eb2525 (red), #a94bfa (purple), #b0a0ee (gray blue), #3595f2 (blue). These were picked pretty quickly, so it might take some tweaking to get colors you like, but somewhere around that level of brightness will be a lot easier to read.
Let's take a look at the content itself!
Reading your blub in the header, it sounds like you're pre-empting why some people might not like your site. There's no real reason to do this. People that like your site and its ideas will join, and people that don't won't, so just focus on all the cool stuff your site does instead. Focusing on progression and growth is a perfectly fine thing and there will be people that are into that kind of thing, so you don't need to explain it. Try to bring those people into your site with your blurb instead of trying to convince the people that aren't interested in progression systems that they're actually really great.
For your quick links, you don't link to your plot or information about quirks, and that information is probably more important than your introduction board and advertising board for new users. Users are lazy, so putting the most important reads where they're front and center and easy to find will increase the odds that it gets read. I found your advertisement board with in about 2 seconds of opening your site, plus I can use CTRL + F to find it because you picked a good name and description for it, so not having it in your quick links should be fine.
That said, I had no trouble finding any of your information. This is strictly a thing for making it easier for guests and new members to find the information, thus increasing the odds that it gets read.
Your rules read fine, nothing wrong there. Though as a note, it wasn't in the rules thread, but you note that 18+ threads should be tagged. Your site is hosted on Proboards, which does not allow 18+ content; all content must be PG-13 and allowing adult content could get your forum shut down, even if all the characters involved are also 18+. You might want to change that rule to something compliant with the Proboards ToS.
Your plot isn't written yet, so I can't talk about that. That's definitely something you should get up ASAP because it looks unprofessional to leave it blank like that. Your sections on the NPCs and timeline are both fine, I can understand the general gist of history, but I don't have much of an idea of what's happening right now. This would make it pretty difficult to write a character and figure out how they fit into the world, and might be a contributing factor to why you're not getting as many people joining as you want. I'm not even sure which city this game takes place in or what that city's like.
If the link to the organizations wasn't in the quick links I never would have found it. Requests isn't a very intuitive place for it. If you want those threads to act as sign up sheets, it might be worth having a single thread with a list of all the organizations and their mission statements in your information board with links to the signups just so they're easy to find. The orgs themselves seem mostly fine. The heroes seem slightly lacking since they're only org is training new heroes instead of one for actually fighting crime. The villain groups motives aren't very clear, which will make it difficult to make characters in line with their motives. Even if you want to keep their true motives a bit shadowy, they must have something they tell the public so they can recruit right? That would be enough of a plot hook to make it easy for people to get involved. The variety and options seem find otherwise if you're not trying to be a school-based MHA site.
And then a few miscellaneous things I noticed when reading the EP thread. In your first section titled "What is EP?" you have a link to what I assume is supposed to be a thread on stats and skills. The link doesn't go anywhere, it just opens the EP thread again. You also say that there's 2 types of EP, and then go to explain what Base EP is. However, you do not explain what Non-Base EP is. Even after reading through the whole thread I'm not sure what you're talking about. There are a few other places that look like there's supposed to be links, but there aren't. There's no link to the "Invasion Appeal" thread in the "Open and Closed Threads" section. Under "Death" you say the template can be found here, but there's no link in here. Same with "Retirement" and "Class Changes." I'm also not really clear on what Base EP is. You say after reaching a milestone it increases, but to what and how and what does this mean? Do you just get a lump sum of EP to spend?
Going to look at your system now. Going in depth on that because, well for one I love systems and I love talking about them, but for two you asked about it specifically. I hadn't read anything about your system up until now, my advice in the other thread you made was 100% about how I went about creating systems and I hadn't read anything about what you currently had at the time, so this will be pure first read feedback on what you currently have.
Before I even dive into the system itself, I just want to state that I'm not a huge fan of pure progression systems in forum RP. I don't think they suit the medium well. This is because people are constantly coming and going and a pure linear path up means that new members won't be able to do anything against older members and this tends to not be fun for anyone (contrast with tabletop RP where pure progression systems work fine because everyone will always be around the same level). I'm probably not the best person to be rating this, so take what I say with a grain of salt. I'm mostly comparing stuff to what I've seen work for other people.
First, I think rewarding people for wordcount is dangerous. It encourages people to fluff up their posts and makes RPing more of a chore than it should be. This is doubly so in my opinion given you have some of your criteria for promotions and exp gains based on writing quality, which is already subjective to begin with. Adds an extra pressure to posting that doesn't really add anything to the story overall. It might be contributing to the low activity you're complaining about. I know the alternative is dealing with spam posters, but when you tie your exp to post count it's kind of unavoidable. In my experience, spamming tends to be better than worrying about wordcount because it makes it more likely that stuff will at least be happening and exp gains becomes more about actually doing things than finding new and unusual ways to fluff up posts, thus making people take forever to post.
The concern I mentioned about pure progression systems before, reading through everything proper it looks like your system has a lot of that. The way you have it set up is that lower level characters (and thus new players) won't be able to do much of anything to higher level characters (established players). On some level this "makes sense," but it tends to be bad for the game overall. It's really discouraging to not be able to do much of anything to affect the plot or current events, because let's be real it's going to be the higher level people driving most of the cool and interesting stuff. Unfortunately, with the way you have things set up I don't think you can fix this either. It'd require an entirely new system, which may not be worth. It's the kind of thing you'd have to talk to your players about and whether they'd even want change because it can also be discouraging to build up something only to lose it because the admin decided to change the system. But I do think this kind of system is only going to get more and more daunting to new people as the site ages.
But moving on, increasing the amount of exp required for level ups as they get to higher levels is a good idea. It gives newer people a chance to catch up, so it somewhat mitigates what I'm talking about above.
You mention masteries by name, but don't talk about what they are and the link to the request form is not there even though it looks like it's supposed to be.
I initially misinterpreted the non-quirk skill option list as the regular skill option list because I can't find what skills do and what bonuses they give anywhere. How do the stages interact with the initial/alpha/delta/omega levels? Do they stack? It is something else entirely?
On the stats themselves, you have 2 stats that do functionally the same thing. Both Defense and Speed reduce the amount of damage you take, down to nothing if they're sufficiently high level. It seems pointless to have both. I'm not actually clear what Capacity does at all for people that don't use it as a replacement for Strength (thus making it so Strength and Capacity are literally the same stat for those people). Trying to model different parts of people doesn't mean much unless they do different things with in the mechanics themselves.
I think the best kinds of systems are the ones that act as writing prompts for the people playing and lead to new and completely unexpected things that the players never would have thought of otherwise happening. This system doesn't really do that. It's just comparing two numbers that are completely public. It looks like the system boils down to compare the strength numbers (Strength or Capacity) against the defense numbers (Defense or Speed) (because people will always use the strategy they have the bigger number for), so the outcome of any fight is basically known before the fight even starts if I'm reading it right. There's no other mechanics to make anything else happen. I think this is the greatest weakness in your system. It's not very complicated, but it's more complicated than a lot of the pure progression systems I see (EG the types that just say you're rank 4 and you need 100 exp to get to rank 5 and largely leave it at that), but doesn't offer that much more to do with in battle than those much simpler pure progression systems. If you're thinking about making any changes to the system (I don't think you need to, I don't think this system will turn people away anymore than any system with stats would), then I think the most important place to start would be to decide what you want your system to do. If you just want pure progression you can simplify it a lot, but if you want people to be able to do weird things in battle you're probably going to have to change a lot.
Moving onto character creation, your process is simple enough. I think having all those appearance fields + a required full appearance description + a FC is a bit redundant, as is including positive and negative personality traits + a full personality description, but the requirements are small enough that it's inoffensive. Your expectations are clear and each field is described well. Including how much exp people start with was a good idea because that kind of thing's easy to forget and not having to look it up speeds creation. Other basics like thread title and what to do when the sheet is finished are also clear and straight-forward. I don't think anyone would have trouble finding the sheet or creating a character.
In conclusion, your site is fine and you're just experiencing part of the normal life cycle of opening a new forum. It's always hard. If anything's holding you back and making it hard for new members to join, I'd say it's not having the plot finished. I don't think your system's going to scare away anyone anymore than any other system with actual numbers would. Unfortunately, stat-based sites tend to be a bit niche in general. I hope all that was helpful. Good luck with your site!
