If you were to find a way to combining Johnson not running for the Tory leadership on a hard Brexit platform with a more moderate Labour that does not scare liberal Tories out of defecting, then I think you could well see a Prime Minister Raab losing Esher and Walton in 2019.
Thatcher would stand a good chance of losing her seat when the Alliance was at it's peak in the polls. It's unlikely she would make it to an election if the Tories continued to do that badly, but pretty much all of the possible candidates to replace her, besides Howe, would stand a chance of losing their seats in a worst case scenario.
Other possibilities I'd add would be Burnham, Cooper or Ed Miliband in some situation where they are saddled with resolving Brexit, which would obviously bring them into conflict with their eurosceptic constituents. Going farther back, there is a decent chance that Tony Benn could have lost his seat had he somehow wound up in Downing Street. His majority was less than 2,000 in 1979, so he was very much vulnerable in the event of a Tory landslide. I seem to recall that there was a chapter in 'The Prime Ministers that never were' anthology that had George Brown's Premiership end with him losing Belper, as he did in OTL 1970.
