I think this would be a ripe situation for Ares. They might back one side, or they might play them both. In the latter case, they'd be unlikely to commit armed personnel to the war efforts, because then you'd have Ares forces fighting Ares forces. What they would offer is bulk arms shipments, training, and consultants. If it turns out to be particularly profitable, I could see them intentionally making minor tactical errors that will see the two sides both making minor victories, but that will stretch out a war as long possible.
As far as actually developing an AI? My understanding is that ever since the SCIRE incident, that is some of the blackest research you can find - no corp wants to create the next Deus, so the trend seems to be reporposes or training "found" AIs. Of course, the megas are really good at assuming they can handle shit, so the creation of a new AI approaching the power of of the old three could be a fantastic behind-the-scenes plot. Programming really isn't their strong suit, though, so maybe the party could be hired to run some extractions against Renraku?