Sonny would serve best as a capo or underboss having someone over him to reel in his anger while being able to put Sonny's talents to good use.

I wonder how Sonny as underboss and Mike as boss would have worked out. I can imagine friction with Sonny taking orders from his little brother but I also think that Sonny could have helped to keep Fredo in line and he wouldn't have had that unfortunate boating accident.

Getting Michael involved was not a bad idea. It may not have been what Vito wanted but it was the only way to go at the time.