All these speculations about what might have happened had Brian not died are very interesting! Regarding the Yoko thing, I've got a gut instinct that they wouldn't have gotten along. I think Brian had a bit of a thing for John and basically idolised him. He could deal with Cynthia because she wasn't a threat and pretty much stayed in the background as the perfect Beatle wife. Yoko on the other hand.... just imagine the frustration he would have felt about John and Yoko's naked album cover and all the controversies he was engaging himself in!
It's also fascinating to speculate ...