Well self restraint doesn't always work, so the characters' base abilities being more limited would keep that in check. I'd like to think everyone has this quality, but also the idea of working with those limits to find more creative solutions to defeating enemies and overcoming obstacles. Maybe it's not as easy a concept to organize as I thought.
I think the problem you're encountering is that too much emphasis is being put on controlling power levels while you simultaneously make exceptions (Luke Cage, Iron Fist) and also poorly define your definition of street-level. Maybe just saying 'nobody with superpowers' would be better, or calling out some specific power thresholds, or even allowing people to tone down characters to fit would work better. You're also adding an additional constraint by focusing on the MCU continuity, which by its nature limits character possibilities because it just doesn't have much of a cast to work with aside from the Avengers. This is an area where some flexibility in thinking would help you a lot - maybe focus on what the characters do
rather than what they are
? There's a pretty good selection of possible street-level heroes that are also incredibly interesting if you dig deep enough. Here's an updated list i got from a few minutes searching google and dredging the depths of my extensive marvel fanboy nerdmemory:Updated list of possible street-level heroes
Black Widow (Yelena belova, the other one)
Echo/Ronin (Maya Lopez) - I think she was already disqualified?
Hawkeye (Kate Bishop)
Spider-Woman (Jessica Drew) - Might be disqualified since she was an Avenger
Silk - Same as Spider-Woman. Seems like the Spiders are right out.
All of the Immortal Weapons: Bride of Nine Spiders, Dog Brother #1, Fat Cobra, Tiger's Beautiful Daughter, The Prince of Orphans
All the Runaways: Sister Grimm, Chase Stein, Arsenic and Old Lace, Molly Hayes/Power princess