One idea I've had for a while is to add a tenth hotbar slot that is always an empty hand, even if your inventory is full. Mostly for mod machinery that requires an empty-hand click to do something, but also just nice to have so you can still have your full 9 items/blocks/whatevers but still switch to an empty hand if you don't want to be holding anything.
Also, a second slot for each piece of armor, so you can wear, for example, a helmet and glasses at the same time. BUT, the second slot would not offer any armor protection, just special abilities like night vision or whatever, to keep people from wearing 2 diamond chestplates and basically being invincible. X3 And perhaps allow wearing of an Electric Jetpack from IC2 along with a Batpack/Energypack that will recharge it, since that thing's power runs out so quickly. Though, I don't know how easy that would be to make work with IC2.