I am not sure its a given that Frank is leaving yet, but if he does, I am going to go with Mike Brown. He is a terrific defensive coach, and might see how Frank and Thibs used the job as a jump-off point to get a head coaching job.
I would love to see Mike Brown take an assistant coaching job under another top NBA Head Coach. He has huge potential as an NBA coach but he needs to refine his skills in a few areas to achieve that.
I hope he doesn't jump into another head coaching job because his career could benefit mightily from putting in a bit more time to learning his craft. He is still very young. Plenty of time and opportunities ahead of him.
Unfortunately, I don't think he'll do this. A pity.
Instead, he'll most likely continue as a mediocre head coach who is ill-suited to leading a top NBA franchise because of his abysmal in-game coaching decisions.