TrIf Pence was complicit and the Democrats take back the House in 2018 then 2020 is the realignment year.
That would be a very interesting TL. At that point, might it be in the Republican Senate's best interest to provide the 2/3rds vote to remove Pence from office and let the Dem Speaker become president in 2019 so that they can run against him/her (it would have to be someone other than Pelosi) in 2020 as a check against a potential Dem trifecta in 2021?
I don't think so. It really depends on if the GOP base likes or dislikes Pence though. If he gets Trumps blessing and the base likes him then they won't impeach him. The GOP does not want to willingly give up power.
The closest to that kind of timeline (I've seen) was the one Spenster was writing but didn't finish :/