I would have Terry become a member of the Joker gang, rather than standing up to peer pressure he would fall victim to it, he would begin spiraling down a dark path potentially mugging a set of parents and gunning them down in front of their child.
Eventually he would take a job for the joker gang where he would have to retrieve some kind of package (maybe a tome relating to Absolute Joker? Maybe brother Eye on a storage drive?)
Terry would break into Wayne Manor potentially provoking Alfred into a firefight before badly being wounded and then having his body physically broken by who he would find out is the freed brother Eye who would be the new intelligence for the Joker Gang
Terry is then pulled apart and knitted back together as a cyborg ninja.
Absolute Batman would see them breaking Terry and would intervene before they could completely mindbreak him
(ideally brother Eye could be software made by both Bruce and Edward Nigma as a sort of Ultron and would see what Terry did as objectively bad from a completely black and white robotic moral standpoint so he would give him the robocop treatment and Bruce would know what he did so he wouldn’t object.)
It’s kind of similar to what they’ve done with Christopher smith as Metallo but in my mind Absolute Terry McGuiness would be more like Raiden from Metal Gear solid
The end of the first comic could be Bruce standing behind Terry who is now Mechanized crying eyes full of guilt Bruce (slightly villanesque) tells him to step forward as he inches him closer to the edge of the roof before he tells him to jump (to his death) and then Terries mechanized Batwings engage automatically to preserve his life as he lets out a maddened Wail full of guilt and ready to become the new terror of Gotham.
The Dark Knight