We should really leave the speculating about his chances until after 2018, don't you think?
Even if Trump is guilty, the investigation will take years, so unless a really massive bombshell drops (and I mean "Trump is literally in a gay relationship with Putin, and selling secrets is his kink" type bombshell), impeachment is not going to occur while Republicans control Congress. And on a related note, the best judge of how Trump is doing with the electorate will be the 2018 congressional elections. If Democrats fail to gain significantly there (which seems pretty damn unlikely, given how A) Traditionally the opposition party gains seats in the midterms, B) Republican Congress seems intent on wasting their majority away on Trumpcare, and C) Trump is not exactly popular at the best of times), then the conversation will turn. If they gain massively, impeachment is possible. If they gain, but only minorly, then real speculation about 2020 will start.
Until then, though, talking about second-terms or impeachment is literally a massive waste of time. It's not even a election year, let alone the election year. Didn't you have enough election last year? Still hungry for more, somehow?