Dems retake both chambers in November. Special Counsel investigation and the congressional probes turn up enough for Trump to either be forced into resignation or removed from office by the end of 2019.
I think folks here are severely underestimating what Mueller might find in the next year and how badly Manafort's trial/Stone and Assange's imminent charges/Cohen's likely cooperation could go for Trump.
It needs to get bad enuf that Republican Senators abandon Trump for him to be forced from office. Even if the Dems win every single Senate seat they possibly can this November, he can't be convicted in an impeachment trial without double digit support from Senate Republicans.