There are 2 things that I feel should possibly be implemented. I apologize in advance if either of these have already been suggested.
1. An award system. This is pretty self explanatory, however, I personally love this feature on other forums. This allows people to work for something. Yes, we already have the level system, which is kind of relevant, but not very entertaining. Plus, this could go hand-in-hand with a previous suggestion of weekly/monthly competitions, etc. At the bare minimum, there could be basic awards like "Kissass", "Donator", "Veteran", etc. You could more than likely get them all custom made for pretty cheap, as the resolution of them are what, like 10x10?
2. Custom group userbars. User groups are obviously a pretty big part of the forum, however, it's rather obvious that currently all people use them for is the title and username color. Until the forum gets bigger, and the groups actually become content and activity based, I feel like custom userbars could be pretty neat. It could also be an incentive towards leveling up. You could unlock the feature by getting a certain forum level, getting a certain amount of group members, having a certain amount of posts in your group's section or something along those lines. Hell, it could be a feature for those who've donated (Although, I know that your goal is to keep everything free on the forum, which I respect immensely.).
So yeah. I know that these have most likely been suggested or considered, however, I thought it'd be worth the mention regardless.
EDIT: Upon further research, my suspicions were proven accurate. Both of these have either been implemented and removed, or suggested.
I kinda prejaculated while suggesting these, so, I guess I probably should have checked beforehand.
Welp, still going to throw it out there that they should be added. My bad.