Maybe do two gaming booster packs. One for FPS, one for fantasy.
Most FPS games have a pretty consistent weapons system.
1 - melee
2 - pistol(s)
3 - SMG(s)
4 - shotgun(s) OR sniper(s)
5 - explosives
6,7,8 - special (could use lightning bolt as a generic)
Those keys would fit 90% of FPS games I've played. Really since those would all be on the number row you'd just need melee, pistol, SMG, shotgun, sniper, grenade or TNT or something, and a few lightning symbols. Don't even bother with the shift key or anything like that, just the weapon selection would be awesome enough for most I think.
The great thing about something like that is it can be marketed to the OCN gaming crowd. You could do a batch of 50 booster packs, then hawk them on OCN and other gaming forums. Might even be able to convince people to get mechanical keyboards, just for the awesome custom keycaps.