What would be cool if they wanted to keep it all in continuity....
Have it so that when merged with the symbiote, it attempted to rewire (to fix) Rage's brain damage... and in doing so, because of the symbiote, makes Rage prone to massive fits of anger (like his name sake). So he would be a little like the Hulk, where he would suddenly become extremely and unreasonably angry - and while doing so, lapses into a "brain damage" state - where he loses some intelligence and just becomes focused on rage, until he's calmed down. The team could come to realize that there's something wrong, and naturally eventually gets the help of Reed and Pym and whoever else, to do this surgery to set Rage right again.