Ummm, conspiring with them to rig the election, for example? There's no evidence that this happened, but there is the fact that he denied having any ties with them, and they, today came out and said they had.
If Bill Clinton can be impeached for lying about a blow job, Trump can be impeached for lying about having ties with with Russia during his political campaign.
Clinton was impeached for perjury and obstruction of justice. Get Trump to testify under oath, catch him in a lie, and you might have something.