Then I want to suggest the "Glass Cannon". It's an energy weapon that costs 100 energy to fire (basically the entire energy tank for most mechs) but does damage similiar to the nuke. The idea behind this is, you have a god-tier weapon but you can fire it only once per room.
I also thought of something like the nanite leeches as energy weapon. Costs 50 energy per shot, maybe more.
And as last, full shield. I think this might be too strong and probably is a bad idea but the idea is it works like the guardian shield but with a full circle (or almost full circle). It drains more energy per second, so you cannot block an entire boss fight but it could help to protect in rooms full of enemies where you cannot track all the different shots that come at you.
I don't have any clue about the health costs actually, so fell fre to decide yourself (if any of those ideas are even acceptable).