Hmm, that is an interesting question. I think Mark would be really, really depressed after having lost Mimi, Collins, and especially Roger. That would probably be the hardest for him. He'd probably sit in a room, turn the lights off, and watch his film he made of his friends to cheer him up. Otherwise, he'd spend his time with Maureen and Joanne if they were open to it.
Maureen and Joanne would be like my aunt and uncle--still together but constantly bickering.]
Roger would die after Mimi, leaving Mark to be the one he says his main good-bye to. It would be very dramatic, with Roger acknowledging how much his perceptions have changed and how he realizes his life was worth living, etc.
I'm not quite sure what Collins would do until he died. Maybe spend a lot of time with Mark? He'd never find love like Angel again--he may just spend the rest of his life remembering and worshipping him/her.
I agree about Benny and the axe
Oh, and one more thing...Mark would finally call his mother one day.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli