How would things have played out in the short and long term? What would have happened to the structure of the Five Families? Would Barzini have outlined a different roadmap than Michael?

Barzini would have killed Michael and Clemenza at that meeting. Over the short term, Tessio would have been in charge of the former Corleone Family, as a near-vassal of Barzini. Over the longer term, Tessio would have to go--no one is more untrustworthy than a caporegime who was ambitious enough to betray his Don for another Don. Barzini would have absorbed the former Tessio family into his own.

Unknown: did Barzini haveambitions in Nevada or Cuba?

The novel says that Barzini had also invested in Las Vegas, Reno and Cuba.