With Gianforte likely to win the special election for the House, he could likely be a main contender for Governor in 2020. And with Zinke gone as the senate candidate against Tester in 2018, and no other republican set to take his place, this is likely Tim Fox's best opportunity. Wouldn't be surprised if he goes for it. Tester vs Fox would be a toss up in my opinion.
Since when is Gianforte a likely win in the special election? Dude got BTFO by Bullock, and he's running against a native son in a Trump special election. I don't think Montanans really like Gianforte very much.