I love the idea of the fire upgrade but it seems like it could be pretty damn OP, I hope it doesn't last long or it consumes energy or what in my opinion the most fun option, make it so that it gets triggered for a short while after killing a few robots in a row like building up a combo meter, I think it could really promote an aspect of killing robots as fast as possible which would make killing a lot of robots very satisfying.
I've been thinking a lot about possible sword upgrades before coming here and I thought adding an upgrade which give you back some energy after every kill. I like to kill robots while using the jetpack a lot so I think that upgrade would make it a lot more interesting. I think if you do find my idea suits your game you shouldn't make it so they give you back more than half a bar of energy.
I think this game is some truly next gen stuff, I've kinda stopped playing other games since I bought this game a while ago and I love it keep it up (especially with more interesting upgrades)