[Lucius Cornelius Sulla]

even include these roles which don't appear in The Hobbit or are only mentioned marginally, like Legolas and Saruman.

This would involve one of the problems with a prequel, because at the time of the Hobbit Saruman would have to be unquestionably virtuous, if not very personable, so that his relationship with Gandalf as seen in the start Fellowship of the Ring would make sense.

[SuziQ]

I don't see how that would be a problem. It would be clear to most everyone going to see the show that the events take place BEFORE LOTR, so seeing how Saruman USED to be before he was corruputed by Sauron.