Button combinations are an idea. I don't think I like it though, at least not for this type of game.
I prefer to keep the interface as simple as possible. More keys and controls are probably fun to some people, but to me it seems an unnecessary complication.
I think rather than button combinations, I'll use equipment combinations. For instance, if you have the same type equipped of torso + arms you can use the grappling hook.
Each item will have a passive ability and an active ability. The abilities are affected not only by which item is equipped but also your energy levels associated with each of the 4 ability categories.
Boots, affect movement: jump height, duration, gravity, acceleration, deceleration, etc. Special abilities give you specialized movement abilities, like a jet blast straight up, or launching yourself as a torpedo horizontally.
bullets have passive abilities like rate of fire, distance bullets travel, bullet force. Activated abilities will be things like homing ability, spread shot.
torso will feature unique abilities like explosives, shields, a drone that can be launched to explore tight spaces.
the helmet features different scanners that can be used to give you information about the environment and reveal things of interest.
I've also thought about a grappling hook. There's a good possibility i'll make that happen.. I'm about 75% done with the character abilities so there's still plenty of room to make adjustments, additions, or replacements.