I think Devin Modster could be a really good QB. But so much depends on development. So far in the Mora era, QB development has been for shit.
Hundley pretty much did all his damage on his own and, as good as he was, he could've been much better. Rosen is good, but he hasn't been developed as well as he should've been. True, the UCLA offense last year was an abortion, and that set him back, not to mention the pourous OL. But one only has to look at the back-up QBs to know that UCLA QBs under Mora's leadership have received shitass coaching.
This new guy, Fisch, is said to be different. He's said to be a very, very good QB developer. I guess we'll see. But I figure he'll be gone after next season when Mora either quits for another job or is shitcanned, so it's probably not going to matter.