I have been playing on ZARP for years, and i remember how fun it was gaining skills for multiple stuff, and couldn't wait for stamina to get to max level, but that took me about few days to get it to max and that's it.
My suggestion is maybe adding some new type of skills (which would be different type of topic and would require more ideas), or adding new levels to the ones that are already there, but to make it much more harder. (requires much much more EXP for next level)
I wouldn't care if it takes me like 100 hours of game time or even more to run and do stuff to gain next level, Also i know there are booster packs and all that for it, but the next level can give you something minimum,(like 0.5 weight in invertory) that wont make big difference, but still rewarding for some players that play here all the time
The stamina skill was just an example, it could be any of those skills.
Small reward with not much difference so the balance of the server doesn't change much
but hard to get to the next level or levels.
Would give the players with lots of gametime to do something more
Cheers!