I think the screwball is the key pitch for Santiago as a starter. It's the pitch that won him the closer job but he eventually lost the feel for it during the season and only had sporadic control of it after that. If he ever gets full command of that pitch, I feel he can have success as a starter since he has a plus fastball and a decent curve.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin