Well I'm gonna fight for it a little
So I believe the main issues are:
* Where would the AI send waves?
* How the heck would you make it so you could win? (lose condition is quite obvious)
* Balancing it with Normal players/without
I think I can solve problem 2 relatively easily: Let players build a fleet. Offer power modules and shipyard ones too, let a starship "dock" with resource nodes to consume them at a high rate (enrages the AI maybe?) or build harvester units which travel back and forth, stuff like that. Problem 1 is solvable from there. Offer various aura modules that would buff allied ships in the area, and/or strong offense modules so you can eventually have an endgame-level fleet (or whatever it takes to snipe the AI) in one ship.
IMO this would increase the available options, so that you can maybe choose to have a Normal player defend key objectives and feed the Champion resources he needs to get to the core worlds and wipe them out, or have a patrolling champion defend a normal going all-in for a giant fleet to cut a swath through to whatever he pleases.
The issues may be bigger though, I don't know :x
I mean, it would definitely require a lot more work to set up, but I think it would be worth it ^^