I think he's fourth in line and would at least be a reasonable, Constitution-respecting POTUS pro-tem and would also probably appoint a reasonable successor (not Marco Rubio, I hope*).
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* As a member of Florida's Congress, Marco Rubio misused his state-issued credit card for personal expenses. It doesn't matter that he paid the balance off -- it's just that he did that in the first place. And then there's the whole matter of him agreeing to be and continuing to be the Secretary of State during Trump's administration.
RE: Marco Rubio or really any members in his Cabinet: what would it take for any of those people to announce -- no matter the consequences -- "I cannot support this jackass any longer." It would be nice to hope they are just giving Trump more rope in the service of a comprehensive and decisive takedown.
Of course, if his Cabinet members all end up in prison, Smiley-face Trump will probably just claim: "Well, I did clean up the swamp at least a little bit, didn't I?"