Most obvious choice would be signing Ryan Tannehill in Free Agency
There was already a logical option for both parties before the hiring of Smith, but now the Ryan Tannehill to Pittsburgh train is full steam ahead with no breaks. It makes sense even without Smith in the picture as Tannehill had the best seasons of his career behind a strong run game. He is a veteran guy who was demoted this past season and could be ok with a backup role.
He has a lot of starting experience in the NFL, and it would make sense for him to be the competition to Pickett for the starting spot. Many will despise this idea, but there are a lot of connections to the Steelers here. Free agency will bring big bags of cash to plenty that hit the open market, but Tannehill should be one of the cheaper options out there.
He will be 36 years old by the time training camp kicks off in late July. That should also see his physical ability dwindle. No matter what, he should be able to bring some experience to a backup role that needs some aid. The connection between Tannehill and Smith is an obvious one. They had some of their best career years together.
Smith was the offensive coordinator of the Tennessee Titans when Tannehill put up career numbers. They were even able to have some competitive playoff battles together too. Not saying that Tannehill will fix the quarterback issue in Pittsburgh, but it would make sense for the Steelers to have interest. If he gets signed this offseason, it has Smith's name written all over it.