I think it's pretty clear that Seth Wallace is the heir apparent.
When the time comes, I think there is a very good chance we end up with an internal hire. Part of it will depend on the most recent success we have been having when KFz makes his decision. If we are coming off a 10-2 season and a possible playoff birth, the likelihood that we hire internally is much higher than if we had, say, limped to a 6-6 finish.
TBH, I think the success of the WBB program could also have an impact. As long as the Jan Jenson hire continues to look like a winner, I think there will remain an appetite within the Athletic Department and much of the fanbase to replicate that success with a similar internal hire.
While a case could be made for Seth Wallace, I think you could also see folks arguing for Lavar Woods, Tim Lester, and even Phil Parker, as each has a decent resume. Perhaps the bigger question would be which coach would retain the bulk of the coaching staff -- which coach would be able to "keep the band together," assuming that is the plan.
Kirk is currently under contract through the 2029 season. But I would be shocked if he's here that long. Nobody coaches to the end of their contract these days, as it becomes difficult to recruit when the world knows your expiration date. Unless, that is, he gets an extension, which at age 70, I think is unlikely. But you never know.