I think Walker's political career is over. The strongest candidates are probably the youngest ones, so Rubio, Cruz, Paul. I wish Paul the best but the Republican base just isn't there for him (meaning they don't like his libertarian-ish style), and that is a fact that is very depressing for a person like me. And good luck to Cruz after that trainwreck of a primary last time. If he ever becomes the nominee (unlikely), I could see Trump not endorsing him as "payback" for him not endorsing Trump during the Republican convention, and that will only hurt and divide the party.
As someone who is deeply involved in my GOP, I can tell you that there is (at least here) a solid base for Rand Paul. Our county GOP chair ran for mayor this year and got endorsed by the Libertarian Party. We love Libertarians. Jason Lewis was also one of the most libertarian Congressmen in the House, and won his primary handily after a Rand Paul endorsement. So he would do well here for sure, but that might not be indicative of greater Minnesota and more importantly Iowa.
I am not totally sure Walker's political career is over. It's definitely in a bad state, but he could run for Congress and honestly after Evers is out he could be Governor again. He was an embattled Governor who lost narrowly in a blue wave year so obviously he has to be at least somewhat popular. Couple that with how Wisconsin is trending red and it's not a stretch at all. 2024 will probably be too soon for him though.
I see Ted Cruz as a really, really weak candidate. He doesn't have the GOP base and has somehow managed to alienate himself to moderates, libertarians, and conservatives in the GOP and only is liked by the few who are in a limbo between there and the few who still identify with the Tea Party.
Marco Rubio reminds me a lot of Richard Nixon. That's all I really have to say about him. He'll probably be President some day and I would bet good money that he has, at the very least, some very shady connections.
Besides those four, I would agree that no one stands out as a solid future candidate, much less a President.