There was a lot made about how much Chip Kelly wanted Marcus Mariota heading up to the 2015 NFL Draft. There were so many rumors out there about what the Eagles were willing to give up to move to the #2 pick. Of course it was all for naught in the end because the Titans fell in love with Mariota and made him the 2nd pick.
So with all of that being said, there is no doubt that Kelly would love to coach Mariota. The firing of Ken Whisenhunt yesterday opens that possibility...ever so slightly. Pro Football Talk has an article this morning about how it would look if the Titans decided to pursue Kelly. The long and short of it is the Titans could work a trade for Kelly with the Eagles. They would have to decide on compensation, and then the Titans could work out a contract with Kelly.
The most recent trade for a head coach was all the way back in 2002 when the Oakland Raiders traded Jon Gruden to the Tampa Bay Buccaneers. The Raiders ended up with two 1st round picks and 2 second round picks in that deal. The Buccaneers ended up with a Lombardi Trophy. A good deal for both sides (although the Raiders didn't do great with the picks).
The Titans would be crazy to give up anything similar to acquire Kelly. While I do think Kelly and Mariota would be able to do great things, the Titans still need to surround.....well you know, Mariota with more weapons. They cannot afford to give up 4 high picks over the next couple of years.
So while it is fun to speculate about a Kelly-Mariota reunion, it is not likely to happen unless the Eagles fire Kelly at the end of the season....which isn't entirely out of the realm of possibilities.