#2. A fresh change to SmackDown Live's tag team division
Not that SmackDown's tag division isn't stacked, but you could argue that they've underutilised talents such as Gallows & Anderson and Sanity, who have basically been non-existent since their debuts on the brand.
Even so, with the tag division centring around The Usos, The New Day and The Bar, it does make sense that WWE would want to freshen things up a bit and add some spice to it. The Miz & Shane McMahon would certainly do that.
#1. A WrestleMania match
This is the most likely scenario. As much as we'd like for The Miz and Shane McMahon to become a tag team and stay that way, we all know that Shane McMahon is always in a featured WrestleMania match.
The Miz doesn't seem to be in the main event scene, so it's likely that The Miz will end up turning on Shane McMahon, leading to a match at the Grandest Stage Of Them All. They would need a good stipulation for the match, such as a street fight.