He'd be better than McMullin(who I don't think could win the primary anyway) But I'm not sure why he'd want it. Wasn't one of his sons considering it some years back? If any of them want a political career this seems like a good option.
Apparently Romney's son is considering a run for Utah Governor in 2020. It is possible that Mitt runs for Senate in 2018 and wins, with his son running for governor in 2020 and winning that.
Which one? Josh, IIRC. That'd work well with the Romney clan, if Mitt runs in '18 and Josh runs in '20, they're both up for re-elect in '24. Mitt can resign, and Josh likely become a Senate lifer.