I'm making a top down boss rush game kind of like Furi but with command inputs for specials like fighting games (such as pressing down, then right, the left, then attack for a special move) and I can't really think of any cool special moves. Right now all I have is pressing down down attack to shoot a projectile, and down right attack for healing. I can't think of much else. Any ideas?

rethink it. 

really. Imagine the actual action of input makes sense. If I [down down], and that shoots in some other direction, does it feel right. But if I [down down] to heal and my avatar does cradle in type motion, that might be better. (assuming I'm not shooting down). I always thought a good use of multi tap was as a skill bonus. You got more points in the end because of your quick combos and not necessarily how you got there. You get to the fight pose (high low lean forward back) with a key sequence as you describe but tapping that last action key multiple times for quick, right there, hits in that pose until a condition was met or removed would add additional user engagement.

As for additional combo ideas of what keys, that's all you Chug. If the user input feels right to the player, the moves are easier to remember.

That makes a lot of sense! This is my first time making this sort of system, so this advice helps a lot! Thank you!