So to make it simpler:
1. Altmeri wizards and powerful Bretons would face the Telvanni .. and win so bad I wouldn't be surprised if half of Vvardenfell (the eastern one) would be erased from the maps.
2. The Legions would flood the mainland and easily win, while Vvardenfell would probably be just de-telvanized and left to rot.
There's no way Morrowind could stand agains the might of a unified Empire. Don't forget that Uriel was no fool. Before his kidnapping he was actually quite militaristic about things, and even afterwards he was able to glue things together.
Of course, should the attempt be made during Tharn's reign then we're in a different situation. Or other emperors for that matter (not all of course, others were capable as well).