I think it would be kind of a waste if they didn't use any known Marvel characters. Certainly there's room for new ones -- who's going to be the next Coulson? -- but it would also be good to tie it into the Marvel Universe by featuring established characters and ideas. There's a wealth of characters and ideas that Marvel still holds the screen rights to and that haven't been seen yet in the MCU -- it would be wasteful not to take advantage of them.
Probably the best thing to happen is that other studios bought X-men and Spider-Man rights away from Marvel. Disney/Mavel is forced to use lesser known heroes. We wouldn't have gotten them otherwise.