Since the ICAO identifiers B737, B738, and B739 are already taken, what will the ICAO identifier for the 737 MAX 7, 8, and 9 be? Would it possibly be B73M7 etc?
The first 7 is omitted and the suffix is now an M for Max, e.g. B37M for the Boeing 737 MAX 7.
Wikipedia has complete list of ICAO codes here.
ICAO Aircraft Type Designators: Search for Boeing and you will find 737 MAX in the list.
Initially they will go with the same designators as the B737NG series. If the performance is very different from the NG, new designators may be chosen. I don't think this will be necessary.
Note that an ICAO aircraft type designator is always 4 characters; B73M7 is too long.