Seeing as you've gone back on what you said previously and decided to add an Orion Drive, would the Daedalus engine be a future consideration?
Also I have a few ideas on possible 'far future' additions!
1. I was reading on the Atomic Rockets website, and it talks about a 'Pulsed Solid-Core Nuclear Thermal Rocket.' I thought this might be a possible addition, given it's high ISP (optimistically 1 000 000 seconds!) , although I have no idea whether this should be in Kerbal Atomics, given it being a nuclear thermal engine, whist also having ridiculous ISP. =P
2. Maybe a thermal engine of some sort. You could toggle which propellant it would use, trading stats like specific impulse, thrust, fuel cost and such depending on which one is used. It would obviously output heat, but given the fact its a thermal engine, I'm not sure as to what extent you'd need to cool it before you start losing thrust and ISP
3. More radiator options! Dusty plasma radiators, maybe? large scale graphene heat radiators to save precious part count on the more heat intensive engines (Frisbee, Cascade, ect) and inline radiators and coolant tanks, which would make using the Ouroboros more effective. Currently, you'd either have to use a lot of radiators or a lot of coolant tanks, which would hurt aerodynamics.
Thanks for reading, btw.