The easy answer is that the Corleone family would break apart, and Clemenza would also be whacked and his operation taken over by Tessio who would pay money to Barzini.

But....here's a fun possibility...

With Michael dead, the assumption would be that the Corleones were extremely weak. In truth they had built up significant strength with the secret Neri regime, about which Tom was aware. Tom also knew that there was somthing afoot after Vito died, because he knew someone would "come at" Michael
he just didn't know who.

Assuming Clemenza could take refuge in the Corleone compound, and further assuming that Hagen, who idolized Michael took a meeting with Neri who encouraged a bold move, it is possible that the Neri and Clemenza regimes could have done exactly what Michael did and knock off Moe Green and the heads of the other families.

IMHO Barzini would have had Tessio wacked the minute Michael died. After all Tessio was not a man of honor, he was a traitor. If he could betray the Corleones, he could betray BArzini. He would have no use for him, and this would have allowed some of Tessio's muscle to come into the Corleone fold and add o their strength.

This would set up a power struggle among Hagen, Neri and Fredo, but I think Hagen would have been able to finesse Fredo by telling him he was his "older brother," and by making him think he was a "co-Don." Neri with his fierce loyalty to the Corleone family and to the memories of Vito and Michael would have been convinced by Tom that with anyone but Tom at the head of the family, the Corleones would lose their political and judicial contacts, and thus Neri would continue in a top role as the major capo regime.