I have no idea whether Heupel would return to Oklahoma (being fired as an OC is likely a wash given his time there as a player), nor do I know whether Oklahoma would seriously look to move on from Venables after one season with a heavily-depleted roster (my guess is that they would not).
What I do know is that, for the first time since Majors and pre-2005 Fulmer, Tennessee is in a position where it's an attractive job. Heupel showed that the program isn't beyond resuscitation and has them knocking on the door of the Top 5 for the first time in almost 20 years, NIL is humming along and fanbase support is blistering as a result of what Danny White has architected. If Heupel were to leave, and that's a big if, this isn't going to be another two decades in the wilderness.