Just had a thought during lunch: how cool would it be if(and don't tell me I'm wrong, this is a hypothetical situation) Portal 2 WAS Half-Life 2 Episode 3? And the co-op was Chell and Gordon working together, both keeping the mechanics from their respective games? Chell can't take much damage(but her low health max is reached by regeneration quickly), but she is a stunt maniac with the ASHPD and can still do some neat things to enemies with portals. Gordon, well, is Gordon, and he takes care of the higher concentrations of enemies while Chell stays behind him. Would have room for some neat puzzles where Gordon basically has to stay on Chell's heels to follow her through the Portals.
Ignore the problems this would have mechanics-wise(i.e. my network connection is too laggy, I don't have anyone to play with, what if I want to play solo...) and focus on if those things weren't problems.