Site that I would love to see but just don't have the time for is a zombie site. Because holy shit that genre needs a revival so badly. And not just a zombie survival RP title slapped on a site with a gritty skin. I mean a full on zombie site. With old school actual mechanics that I'll explain:

1. Hybrid time (The day IRL is the day in RP but that whole day is on fluid time) 
2. Small world events happening on certain days in certain areas. (and staff dropping those things into your threads)
   
3. Character death on by default (Because if your character does stupid things, they should win stupid prizes. hard stop.)
4. Scavenging mechanics. (dice rolls or a point shop, whatever. Just something that limits players from saying they have like; a shotgun with so much ammo)
5. Equipment management. (Like if you do have guns, ammo counts and things)
   
6. Infection mechanics (Like if your character get's bitten they turn after x amount of time or threads, etc.)

The problem is though, this is a lot of work on staff. Especially with the small world events, like staff would have to go through and have a table to reference on what days and in what boards hordes would be around. Members would need to understand that thread crashing would and should be expected with regard to staff dropping zombies and raiders and stuff into your threads if your thread is on a specific board on a specific day. etc. It's, a whole lot of moderation and Dungeon master work for staff honestly. Like you'd need a huge staff team, or at least a staff team for your specific; typical staff work things, and then a team of mods/global mod like story teller roles that act as Dungeon Masters essentially.