That was the question recently asked on Quora, to which my answer was:

What seats do you think the Liberal Democrats will gain and lose in the 2020 election?

The most promising territory currently looks to be areas which voted Remain (as that’s the big distinctive issue for the Lib Dems) and which are held by the Conservatives (as it’s almost always much easier for a party out of power to take seats from a party in power rather than from another party out of power).

That would match up with where the Lib Dems are currently doing well in local council elections which is not a sure predictor but a useful indicator.