The other way it could go is Mueller delivers a secret file of his findings with respect to Trump’s culpability for crimes to Congress. Obviously there would not be an impeachment vote on it while the GOP controls the House (but the GOPe could use it as leverage against Trump to get amnesty, keep the ‘free’ trade agreements in place and kill the Wall).
That way the Democrats could leak stuff about Trump said to be from the secret file (or just make it up) with the help of the lunatic MSM in the run-up to the 2018 election. Lots of opportunity for them to create October Surprises from the secret file.
At some point, Mueller is going to have to release a public report.
It seems to be becoming accepted wisdom that Mueller has shifted gears from collusion, (whatever that means) to obstruction. If all he comes up with is an obstruction charge for referral to Congress, he has a new problem. Trump’s popularity is rising at a good clip. In the end, impeachment is political.