Not to discourage the Dev's looking at this, because of course they could do it better and smoother, but a hard ai mod is possible.
There are several things that come to mind from the suggestions, probably starting with the militia and pirate fleets getting set to playable = false. The pirates of course are that way to start with, but the militia pulls from the tech setup, so it would mean creating a tech2 or even a militia setup, make it unplayable, and tweak the stats accordingly, faster build times, heavier weapons, or higher weapon upgrade rates, lower research costs and lower ship costs.
Perhaps higher upgrades in bonus percentages as they go up for militia planets, heavier armor.... etc.
While i have noticed the ai to run sometimes, perhaps even alot, i have thought this was because of a "outnumbered" or "out-performed" level in the game that looks at the strength of the fleet against the opponent. Lowering costs to output more militia ships (or Vasari or Tech or Advent as the unplayable race) means more ships, but all of that can be done now, though certainly time consuming to go through the ships, abilities, planetmodules and research and the galaxydef and gameplay.constants files.
Still doable though.
basically recreating a "complete" set of races as unplayable alternates used as the ai races. Unless i am missing something here. Can we designate the ai race in galaxydef? I know militia and pirates can be tweaked there. Or the constants file? Hmmmm..... i will have to check.
In addition, some of the mods already available change the gameplay tremendously. Depending on how difficult you want it.
Kiedjor's Epic mod for Vanilla and Unknown Darkness mod for Entrenchment show a great deal of really exciting opening with militia and pirates and opponent races attacking from the moment the screen opens. Tons of ships and a major fight right off the bat. Haha, i played the first hour solid without getting off my home planet because there was no let up in the attacks, it was fight/defend solid for over an hour. Haha, sorry to get away from the main ideas, but it seemed appropriate to show that hard ai can be done now with some creative and interesting tweaks.
Kiedjor uses what he is calling the "Battle System" and its not my place to tell how he does it, but it is interesting, and if you look at the mod it is there to be seen how. Creative uses of abilities and costs and in some cases giving abilities to the ai races that until now have been for the main races only.
Anyway, it is worth the effort, in my opinion, but again, not as polished or as smooth or even perhaps as nice, as the Dev's would do it. But still doable.
Just some thoughts,
I have to run off now for class, but maybe this weekend i can get some time in to try some of the ideas here to get a Hard AI Mod in the works.
And Happy Gaming and Modding!!!!