We ND fans know that all too well.
It's a roll of the dice.
I think Kelly will be at ND for a few more years, so this is really a wasted effort, which makes it a perfect discussion on a fan message board.
However, if I'm Jack Swarbrick, and BK comes to me tomorrow to tell me that 2019 is his last year, I would start my search with:
Urban Meyer - I highly doubt ND would be able to overlook his ethical lapses, but IF ND decided that, damn it all, it's time to win a national title again, then this is the obvious choice. Maybe ND could put guardrails in place to ensure he stays on the straight and narrow. Of course his health is also an issue. I think he would give it strong consideration.
After Urbie, I focus on these four:
Matt Campbell
Luke Fickell
Pat Fitzgerald
Brent Venables