That would be huge if it happened, but highly unlikely. EA makes its money by producing multiplatform games. Microsoft would either be forced to produce games for competitors hardware or throw away 4/5ths of EA's potential income which would make the company worthless. Granted EA does make some exclusives, but Microsoft would be better off buying a highly talented studio that focuses on 1 platform at a time. So a company like Level 5 would be a better choice.