Well, he's violated the emoluments clause already. He also has something like 70 open lawsuits which could yield impeachable offenses and, of course, if there's any collusion found with Russia it would be all over. Except for the fact that Congress needs to take that step. Will that ever happen? Anyone's guess.
@johnny253: The guts of it reads, "no person holding any office of profit or trust under them, shall, without the consent of the Congress, accept of any present, emolument, office or title, of any kind whatever, from any king, prince or foreign state." An example is the Kuwaiti Embassy recently moving an event scheduled to take place at the Four Seasons to a Trump hotel.