I would put some small time, unknown heroes/villains in this show to keep people interested. Somewhat like Arrow, I do think it is the cameos that keeps some people viewing week after week.
The problem might be the character licensing, I am not sure how Sony owning Spiderman world and Fox owning the XMen world affects who they can put on the show but characters like Moonknight, Prowler, Taskmaster, etc... are some that I'd like to see, not exactly obscure but still relatively unknown to the public.
See thats the tricky thing... Those companies own the movie rights, this is tv but this is apart of the cinematic universe. I tweeted this exact thing to AgentM (editor at marvel) and he was being dodge as hell about answering this.