I agree with this, but it shouldn't be based on "Nithiem Rebels" or those other random faction names. Instead this feature should replace our current "economist/aggressor/researcher/fortifier" set we have going. There isn't a major difference in behavior or overall strategy between these four, they just have slightly different unit priorities. If I set the behavior to random (and I usually do) I actually have a very tough time figuring out which kind I was up against.
My suggestions of replacements would be the following:
Expansionist
This would be the replacement for the aggressor. The expansionist desires to build the largest empire possible, and simply expects that a strong economy will follow if it does this. The expansionist maintains a large fleet size to protect and further expand its holdings. It aggressively attacks neighbours who do not fortify their own planets effectively, but spends little on defenses itself, mainly relying on repair bays and frigate factories to supplement its fleet.
The expansionist AI is all about momentum, pushing violently against its opponent and looking for easy prey. It generally avoids expensive civic technologies, preferring to expend its money on high-end military tech. Diplomatically the expanionist prefers to make allies with more distant empires, preferring to view its nearest neighbours as its next meal.
Planner
The planner is a more subtle AI that replaces the fortifier. In many ways it's the opposite of the expanionist. The planner will try to establish and consolidate a lean and stable empire, build up a powerful economy, then annihilate its foes. The planner will expand its empire during the early game to a reasonable size, but will then set up defenses at a sensible choke point. It will then focus on building a powerful economy until it can crush its opponents.
The planner is about biding its time, building a suitable foundation and then waiting for it to pay off. The planner will keep both its military and economy lean until it's ready to boom, at which point it will hunker down and start economizing. Once its got a kick-ass economy, it opens the proverbial gates. The planner prefers to make allies with its neighbours, preferring the safety of being in a pocket (if it can...)
Raider
The raider is about hit and run tactics. Its entire strategy revolves around wearing down an opponent and avoiding direct confrontations. The raider uses a combination of very offensive and very defensive strategies to achieve this. It will send in raiding parties - carriers laden with bombers, for instance - to destroy enemy assets and then retreat when its fleet arrives. They will then fall back to starbased areas to reinforce.
The raider is all about keeping up a pressure game, preferring to distract an enemy across a large reach of space rather than fight him in a localized battle. The raider is therefor less effective in more confined areas, and would prefer to make peace with its more nearby neighbours to attack its more distant opponents. The raider is not above making a full-force push as a coup-de-grace.
I think those are a little ambitious, but hey, a guy can dream 
