There would definitely have been a rubber match between Muhammad and Joe. Maybe not in Manilla,but somewhere. Ken Norton may have still upset Ali in a title defense scenario. There'd have been a return bout,and Muhammad would have become three time champion earlier than he did in real time.
Alis career might take a slight hit with Foreman missing on there. Fraziers career might look a little bit better with only 1-2 losses to Ali. Fast Forward to the 90s Holyfield wouldnt have anyone to defence in his first title defence and may have fought Tyson instead. So we could have seen Tyson-Holyfield in 91 Moorer wouldnt have lost his titles so we could have seen Moorer-Holyfield 2 earlier, or maybe Moorer-Bowe/Lewis.