3) As A Villain
As of right now, Agents of S.H.I.E.L.D. is using Ghost Rider as a reluctant hero of sorts, but you know that it won’t be long until he’s working alongside Agent Coulson and company. The interesting thing about the character in the comic books though is that he often becomes a full fledged villain (it’s not easy maintaining control over a literal Spirit of Vengeance to be fair), and regardless of whether it was in a follow-up to The Defenders or a big screen property like Doctor Strange, Marvel could have had a lot of fun with Ghost Rider by making him a bad guy.
Some fans wouldn’t have liked that, but the door could have been left open to him one day finding redemption. However, as a villain, that would have marked an exciting and drastically different use for Ghost Rider.