I am 90% sure that Sullivan will not be fired.
I am 75% sure that Sullivan will be back behind the Penguins bench next season. I think it's possible, but still unlikely, that Sullivan forgoes his contract extension, steps down, and looks for a new environment.
If Sullivan returns, I am 99% sure that there will be at least one new assistant coach on his staff. Reirden and Vellucci both have expiring contracts. It's possible that one or both will not return. And I wonder about Ty Hennes' value and role, too.