New characters seemed to be inevitable for a few roles. I think I suggested as such, I think. But a healthy mix of new and unused B/C character roles from Marvel's library seems also inevitable...if the show survives.
I don't think it would hold fandom's interest full time or with solid numbers. Nor do I feel the regular tv viewing audience wants to see CSI: SHIELD.
The new characters allow the writers freedom to do nearly anything from a drama perspective. Who's in danger, a traitor, fall's in love, dies etc, while bringing in a Mockingbird type character or other B tier Avenger gives the show that bit of something unique.