It would be helpful if filters were more flexible and powerful by being able to individually modify certain fields by filtering by what something is not. (Particularly for just taking specific titles out of certain filters) For instance I might want all of my games that aren't completed for a sort of backlog of games, but that wouldn't include games that don't have any real goal, so I might want to add "NOT genre: sandbox" to the filters.
In addition filtering by custom fields (which i understand is on Jason's radar from reading a different post) seems to me to be a must for any kind of custom filters, and I'm sure the usefullness of filtering what something is NOT will only become more powerful with more things to filter by.
Finally, an easier menu for mass adjusting of fields (some of the databases aren't quite right/setting custom fields for larger libraries is a hassle whenever custom fields become useful) than opening the settings for each game individually and navigating to the fields and then setting them seems necessary. In my mind this would be more of a checklist of games to add/modify at a time, and then the operation to perform (add genre, add playmode, set playmode, set genre, set custom field, add to custom field, etc.)
The difference between adding and setting would be very important as setting could mess up other filters/narrow the genre or playmode of a game too harshly (some games are multiplayer and singleplayer, horror and fps, etc.)
Since the two biggest features of the premium launchbox IMO are the custom filters and the theater view, it seems like custom filters need a bit more power.