Here's some thoughts:
1) screen real estate could be used better. How about moving Game/Version Info to a box in the bottom left like itunes/iphoto? This would fill some of them empty space in the collections column, and allow to use the place currently holding one screen-shot to show more screen shots at once in a row
2) cover-flow...
3) small buttons for tv filter/color correction, or even better, move them to preferences window
4) use internal sid player, no need to open separate sidplayer (altough I like that very much!!!
5) smart folders...
6) some place to store docs for a game
7) support for other gamebase databases (amiga etc.)
remember view settings between collections (ie list for the full database, grid for favorites etc.)
9) rating mechanism
10) play count/time
11) handle game files stored as extras: double clicking original tape should start it in vice, not unzip it
12) full screen mode (maybe with original c64 font or something cheesy like that)
13) multicolumn filter like itunes (ie first select year, then genre, then author etc.)
14) links to zzap64 reviews
that's about it for now