Wow, fantastic engine. As more of an artist than a programmer, this is exactly what I've been looking for.
I see that you're open to taking suggestions! I read that you added melee attack, which is awesome. What about acquirable sub-weapons like in the Castlevania series? So a gamemaker could have his character use melee attacks (or the bow) on a normal basis but then have projectile (or plantable, like a bomb) subweapons. Maybe even have an option for the subweapons to rely on 'MP' or 'heart points' like in Castlevania which can be replenished by another type of heart item.
I know that's likely a lot to program and you probably have a lot on your plate, but I thought no harm done to suggest a couple of ideas!
Again, amaaaazing engine! And super sexy pixel art, haha.