I think/hope House Democrats are smart enough not to waste their time on this (unless we've reached a scenario where a significant number of congressional Republicans support impeachment), and they instead focus on beating Trump in 2020.
If Trump does win re-election, though, I could see him being impeached if Democrats control the House. Scandals tend to pile up in the second term, and I suspect that this administration will have had more scandals than most by the end of year four.
They have more scandals than most RIGHT NOW