Good suggestions in your post. I’m half of a father-and-son GSB duo, and my kid has been asking me increasingly pointed questions about if the game will eventually have reinforcements. Of course I have no clue, and I tell him so, but now we’re studying the subject in detail W.R.T. game balance. I happen to like the notion of staggered deployments; not Survival mode, mind you; just a small group of fresh, unscarred units to gallop over the hill and try to pull one’s metaphoric chestnuts out of the fire.
I think that a late-arriving combat group should by no means constitute an absolute guarantee that if you were losing badly, thou shalt now win handily. It should extend your chances of such without automatically being a deus ex machina. For smaller battles, perhaps a single fighter squadron or a trio of frigates. For larger battles, perhaps a pair of light cruisers led by a heavy cruiser, with a couple of fighter squads (one for escort, one for anti-ship assault). I concede that’s a tricky road ahead to add suspense and fun while not wrecking game balance utterly. But hey, that’s why we debate these points on cliffski’s nice forum here.
If you could specify the exact composition of your reinforcing group but have it stuck on a variable timer where all you can do is choose the minimum interval prior to which they cannot show up, that also could be fun. I like the semi-random nature of that. Or perhaps the opposite - pick exactly how many minutes & seconds until reinforcements show up, but their onboard mix of modules is randomized by the game program.
Again, if this is ever implemented, it has to be sufficiently different from Survival mode to still be enjoyable on its own merits.