Why hasn't Donnatello attempt to use his newest Retromutagen formula on him, is it possible Timothy is mentally and biologically beyond help? Also what's interesting is that both Timothy and Spike acted as companion cube's for both Donnie and Raph who vented their negative feelings out to them. Both misunderstood and took action once mutated (or in timothy's case mutated further). While Spike reformed and leads the mighty mutanimals, Timothy is permanently trapped for now frozen. So will we see him get a heel face turn.
Timothy is still frozen, however considering he is an ally of the turtles in most forms of media he might reform and be good. Though he might still be mad at the turtles and Donnie. Also once he founds out about april and casey he might get even more vindictive. Though slash took a couple episodes for him to get a heel-face turn and become an ally of the turtles. We might see that for timothy. I doubt they forget him like Pizza cat.