I thought of another character since making my first post of possibilities but I think he should write Superwoman. I am not talking bout the current Lane Lang book but instead the Kristin Wells from Pre-Crisis or Lucy Lane since she was the last to wear the costume during the War of Supermen. He has shown he can write woman so this would be could to build from the ground up.
I truely hope Bendis is restricted to Vertigo and Earth One's stories (even if I don't belive that will happen) for I can't think of a less suited writer for DC's current Rebirth initiative. Bendis biggest problem is not even his lack of continuity knowledge (or his caring less about it), for continuity at the DCU is really fluid at the moment, but his awful lack of consistency in writing. The way he simply adapts characters to whatever "cool" story he has in mind, totally disregarding said characters basic traits and motivations. As such, let him have free reign with Earth One (as he did so well with the Ultimate U), or with Vertigo's self contained universes.
Peace
I think I’d be quite enthusiastic about idea of him doing equivalent of Ultimate Spider-man..a long, long run on a character set on a separate Earth, unconstrained by main DCU continuity. It would play to his strengths I think.
Not Batman...there’s already so much strong material there..that it seems a wee bit redundant. Not Superman...I’ve already admitted my bias that I don’t think Bendis is good on ultra-powerful heroes.
Hawkman perhaps?
I think Bendis would be good at tackling Firestorm since he wrote Ulitmate Spider-Man. Ronnie/Firestorm was basically conceived as DC's answer to Peter Parker/Spider-Man. Bendis would really excel at tedious back and forth dialogue between Ronnie and Stein inside the Firestorm matrix.
I think the last thing on Bendis' mind is doing Ultimate Spider-Man 2.0.
I wouldnt be surprised if they let Bendis make a character who was insert-DC-character's classmate in high school, unmentioned until now best friend, had a secret history of superheroing, got molested and then some.
I actually really want to read a solo for Vixen. There's not much content for her, I do like her in the JLA. I'd like to see Bendis have a crack at her. If only to raise her profile so that other writers would want to use her more.