At the time, I was suggesting that Fortresses being allowed to use warps, but require supply.
That was the first idea that came to my mind in response to MaxAstro's point. I'm concerned about units that magically don't work based on where they are, but "units that only work on your planets" makes a lot more sense than "units that only work when not on your planets", so I think it's doable. It can fly anywhere, but the guns don't work unless the controller is friendly. If players abuse them as HP sponges on AI planets we'll deal with that later.
The fortresses would need to have a galaxy-wide-cap, and not cost power (because they can hop planets, and no unit that uses power can do that as it would require a blackout mechanic). I'd prefer that they not cost fuel either, as that would compete with offensive units for that resource (the whole idea of the fuel/power split is to avoid that).
So they would be a straight up exchange of permanent science and temporary metal (perhaps a
lot of metal) for a mobile defensive bruiser.
But they wouldn't exactly be a rapid-response force. You could still get caught out of position.
As an alternate, a drone-based "Strategic Reserve" - You build into the reserve using resources (and perhaps build structures that require power/fuel to increase the size-of) and those reserves auto-deploy based on AI strength when a Human system is attacked. Humans cannot control the units, and they won't ever go on the attack, but it is a global "help on defense" while at the same time not being too fidgety or micromanaged.
I like this a lot, because I want something that doesn't require player attention to do a decent (if not optimal) job. Even if you don't have to bring your fleet back, bringing your tactical attention back is almost as bad.
So:
1) Add "Warp Drone Cluster" to the Infrastructure build menu.
- costs significant power.
2) Give the Power Distribution Nodes (or whatever guard-post-like thing they become) the same mechanic the Carrier Starship uses to automatically produce, store, and launch its drones.
3) Each of your Warp Drone Clusters, anywhere in the galaxy, contributes evenly to the drone production of all your non-full Power Distribution Nodes.
- so if only a few nodes are not at their drone capacity, all the clusters will focus on those few nodes
- unlike normal carrier-drone production, this extra production should cost metal
Done
Well, except for selecting which drone types are available and whether it's tied to fleet ship tech, etc.
One possible simplification is to not add "Warp Drone Cluster" and instead simply give that functionality to the existing Space Dock (the downside is that it's not obvious the space dock does this). It would have the same effect: you probably aren't building these on chokepoint worlds (too much power), but you can put them on interior planets to support your defenses anywhere. It weakens the defenses you can put in that interior, though.
So fortresses would be the big-doorstopper that you have to position manually that costs lots of science and up-front metal, and warp-drones would be the "fire and forget" option that costs lots of territory and during-battle metal. The latter would be less likely to actually stop an AI attack, but they'd slow it down.
Players could specialize in one, both, or neither.
Worth thinking about, anyway.