I'M ONLY FAMILIAR WITH THE THIRD OPTION, or at least...I can't guess the other ones.

Gokudera as a pissy werewolf with explosives would be AMAZING. I couldn't offer Stiles as anyone competent to deal with him, but they'd butt heads gloriously. BUT the first option for the child-immortal vamp sounds pretty awesome too... I CAN SEE WHY YOU'RE HAVING TROUBLE PICKING, too many options...