I don't believe there is really anything to impeach about. If all Comey's memo says is that Trump said "Flynn's a good guy, I hope you can let this go," then I don't think that's enough. If there is more direct evidence of obstruction in the memos, then maybe there's something there but it also brings up the idea that if Comey thought there was obstruction going on, he had a duty to report it when it was happening.
I've heard the theory that obstruction would be evidenced by Trump firing Comey, however, McCabe very clearly said that the investigation was not impeded by this. I guess maybe you could make the case that it was Trump's intention to impede the investigation, however Rosenstein, the same guy who called for Special Counsel, did write a memo to Trump recommending the removal of Comey. It isn't so clear cut.
If he's going to be impeached, it will have to be for something we don't know about yet, like there's evidence he actually colluded with Russia, took bribes, etc. But that's going to take awhile to develop now that there is Special Counsel investigating.