It's well known that in the beginning of his career, The Miz was bullied by John Bradshaw Layfield. In fact, throughout his run as WWE Champion, Miz brought that up on a number of occasions. Based on what I've read and heard about Layfield over the years, I can only imagine what kind of hazing he put The Miz through. I'm sure it wasn't pretty.
Given their history, I think a match between the two would help bring Miz full circle in terms of career growth. A feud with JBL could either turn him into a face, which the company could build upon for the future, or revitalize his role as a top heel.
A scenario could be set up where JBL takes credit for making Miz the superstar he is today and, wanting him to acknowledge that, rubs Miz the wrong way. Another scenario could see Miz wanting revenge and attacking JBL, remembering the pain and suffering he was caused. Of course, these scenarios would have to be fleshed out more for better effect, but you get the gist of what would be accomplished; Miz would definitely get his mojo back in the eyes of the fans.
Of course, this all depends on JBL and the kind of shape he is in right now. It's been a long time since he's been active as an in-ring performer. Can he still go or is it best that he stay retired?
In an ideal world, it would be great if JBL could still perform. He would definitely kick the crap out of The Miz, perhaps toughening him up. This match would be the shot in the arm Miz needs in order to be relevant again. Even if JBL is no longer capable of performing, a feud involving the two would still be compelling. You could always have someone step in on behalf of JBL if the goal is to turn Miz into a face; with his money, he could hire any number of guys to take Miz out. He could place a bounty on the Miz. If done right, this storyline could become compelling enough to last a good part of the year. Miz could become an anti-hero like so many others before him.
The possibilities are endless, of course, but the impact would be the same—propelling Miz back to the top of the company. He's been experiencing one hell of a slump lately and quite frankly, he needs something to get back on track. JBL can be the catalyst for Miz's turn around. Look at what he did for John Cena's career.