1) A new "lounge"-type forum section, or sub-forum in the "General" category. In the absence of a chat room it might be nice to have an area for non-DK related topics or a place for people to hang out when they're just lurking this site...as I often do.
2) Some stickies in the several sub-forums to help new players or players new to streaming. For example, how to deal with MAME/romsets, how to set up a quality Twitch stream (settings, hardware advice, etc), perhaps some stickies on DK cab repair/maintenance. I'll admit I'm no pro at these various things so it would probably take a community effort.
3) Corey's great effort in compiling the high score list makes me think that DK could use some more quality historical archiving. So, even though this is a forum site, it might be nice to have an "About Donkey Kong" sort of page, and pages or threads that explain the histories of the the DK sequels, clones, related games, etc.
As you can tell, these things are probably more interesting to players new to DK and new to the DK community, so I wouldn't be bothered at all if they don't fly!