There are two possible avenues for impeachment:
1: If Trump obstructed justice (this is what Nixon got kicked out of office for)
2: If Trump broke the law
The first one, to me, seems to be blatantly obvious. But we'll have to see what Mueller says. Concerning the second one, "collusion" with Russia could potentially break several laws including:
- Accepting a contribution from a foreign government (52 USC 30121)
- Gathering or delivering defense information to aid a foreign government (18 US Code 794)
- Conspiracy to defraud the United States (18 U.S. Code § 371)
- Fraud by wire, radio, or TV (18 USC 1343)
- Benefiting from corrupt foreign practices (15 U.S.C. § 78dd-1)
- Failure to disclose campaign finances ( The Federal Election Campaign Act)
If Muller brings evidence for either of this claims I think impeachment proceedings will begin. And if Democrats take the Senate than I have no doubt Trump will be impeached. Otherwise, with Republican's in power I'd say the odds are 50/50. But all this really hinges on what the Mueller investigation turns up.
We'll have to wait and see.