I like the defense Nemesis idea better. The idea of it more directly attempts to prevent the single HW blitz.
What I wonder though, is how to implement it.
Imagine the end the of the nebula campagin the offensive tools you get:
-A champion that is in combat almost a mini golem. Respawns instantly.
-If you do OK and not good on the nebulae you get at least 10 stations. I had 14 but we'll go with 10.
--You get (7 * 10) starships, a whopping 70 starships.
So the nemesis, if it wanted to stop a HW blitz, needs to stop the BB and the 70 starships. My question is this:
If the nemesis can stop all this, what else can the player get in the base game to overcome it? If you took 5 ars worlds you'd get around an additional 1000 fleetships or so. That is significant, yes. But would it be enough to stop the nemsis? Does the power of 1000 fleetships make that much more of a difference when you have something that can stop a mini-golem and more starships then you could hope to field on your own? You'd have to do an all out assault to knock out the ai, and the brutal picks on the table today will devastate such a tactic.
What I am trying to say is that such a tactic will make it so all HW assualts will be a slugfest where you must attempt to smash the ai with the largest hammer ever in order to hopefully defeat a shield that can can stop what is your already large hammer.
Timers or no, such an all out assault would take over an hour to recover from. Even just replacing the 70 starships takes over one half hour for me, and that is fine, but if the thing they were trying to stop has respawned...its just a stalemeate.
However! I do like the idea. I just think there needs to be an additional feature to make it work. How about this:
Make it so you have several Nemesis on the AI HW. 6 actually or so. Each one can't take on the BB and its fleet, but 2 is a costly fight, and 3 drastically, does so, and 4 and above is for cheese masters. Each of these nemesis, however, require a "respawn supply structure" that is spawned similar to CSG logic but is an independent building.
In practice, what this means is if you try to take the HW without taking some of those respawners, then you will struggle to knock out any of the nemesis, and the ones you do come back. However, if you were to knock out most of them, then after their defeat most would not come back. You would have one resupply structure on the AI HW itself so one will always remain, but one is not enough to stop the fleet but be a continuous nuisance as it respawns.
This would enable to accomplish the goal of preventing a single HW blitz, but with the added layer if you pursue the alternative of taking key worlds (they are key, since they are using the ars layer of CSG's) then you get the benefit of also knocking out the AI worlds defenses.
This I feel would accomplish the desired goals even more strongly, and not be quite as a knight of trying to balance the defense that needs to stop an super mighty single world offense but still be able to fall to an only slightly larger offense.