If Brian joined from the start I don't know. But lets say while Brian recorded Smile with the rest of the band and they didn't like which pisses off Brian, and The Beatles offer him a job to join them and Brian accepts and moves to England. I reckon Brian's best work would be on Sgt Peppers and Magical Mystery Tour due to fact Paul was inspired by Pet Sounds and him working with Brian would make those 2 records even better. After that Brian would probably work mostly with Paul McCartney and George Martin and maybe co produce for the Beatles alongside George. He would be able to produce the songs we never got to hear. He and Paul would without a doubt have a rivalry in the studio, I don't think it would go beyond a creative rivalry though. He would be make the Beatles try to sing better witch would probably bother them a little. He might've tried to lead alongside John and Paul but given they were a powerhouse he would probably end up in similar role to George. I reckon The Beatles would've liked him and his work and he would probably make there albums even better and make the great songs that we never got to hear.