it's hard for me to think about without making the roleplay like a game. but i think a feature that would be attractive to me is community goals. these goals would contribute to adding a feature to town that wasn't there before. these might enable a new event to occur or a new shop to open up. for example:

- some projects might require characters to donate straight currency. like the town building a museum and that introducing artifact hunting or just... a compendium that characters can contribute to.
- some might require an amount of specific items bought from the store for a recipe. this might unlock a feature that makes town life easier or a new barn/coop animal to collect.
- others could be unlocked through a certain amount of posts in a thread. ie. everyone posting in a community garden thread and at the end of the month, earning crops/items to use or sell. even just site currency.

not sure on how to incentivize sole character upgrade purchases w/o having game mechanics but i do believe community contributions of some kind would be neat to see.

not quite what you're asking, but to offer my own perspective, mechanics aren't really what keep me on a site.

events that alter the setting do.

even in harvest moon, you're playing the game with a purpose beyond yourself. there's a town to restore, a goddess to revive, or broken relationships between towns that need mending. what helps most is to have a goal, and to see payoff for that goal.

events that would keep me around would be things like preparing for a storm, repairing damage from a natural disaster, tracking down a potential "thief," or trying to find a hidden treasure somewhere within an old town.

i recall one site i had joined a while back that would have different disasters that would be isolated to specific housing districts. setting up for people to be able to participate in threads centered around a power outages, helping friends clean up after the river flooded their homes, or looking for lost sheep after a forest fire saw them scatter to the woods would all be great prompts that i'd be interested in taking part in and could definitely help to make a character feel like part of a greater community.

while leveling up tools and crops gives you personal goals to work toward, if it's something that isn't leaving an impact on the site, it can lead to feelings of isolation. shonen sites often have power ups for purchase, but suffer from a similar problem. the setting itself is static, the rewards are geared toward the individual, and in the end - you're left with nowhere to grow.