The only downside I see to portal is if you do use GLaDOS, everyone has to be a mute because all you need to do is talk to her to kill her and bam, RP over.

Or pull a futurama and she has paradox absorbing crumple zones. xD I'll let you guess where she has them.

Actually if you have a video game crossover rp sorta thing in Aperture that could be fun. Mario and friends doing Aperture tests might drum up interest.