#It's probably overpowered
The thing with shields is that they add a big boost to AC at the expense of increased damage. Usually the shield prevents you from holding two-handed weapons which have the bigger damage die - or prevent you from holding a second weapon for two weapon fighting.
Turning that into another attack option skews this to something everyone is going to want to have.
+2 AC (or more!) and I can still attack with that arm? AND I get the other shield master attributes? YES, PLEASE!