What I hope happens is something similar to the Suicide Mission but taking your choices from the game throughout to determine the Final Battle.
Examples:
-Save Rachni, have a surprise attack that gives the Normandy a clean shot at Harbringer. They come in at the last second to flank the Reaper fleet from a dormant relay at the battle spacefield lol
-Save Council and destroy base, end up getting the turian fleet to help you, which is supposedly gigantic. Alliance cooperates but only after Cerberus is put down after they attempt to convince the Alliance they need them.
-If you let them die and saved the base for Cerberus, the events of the story give Alliance fleet time to upgrade their weapons and armor on their most capable warships, negotiating (with brute force) further domination on the Council's front. Cerberus becomes heavily involved with Alliance again.
-Save Council and base, you end up having to put down TIM and turn the base over the Council, who only has time to upgrade the Ascension. TIM wants humanity to rise to the top, and the Council sure as hell won't let that happen.
-Mordin can start working on genophage cure if you kept it, which combined with Grunt's Rite of Passage and whether or not Wrex lived, allows the increasingly unified male portion of Tuchanka to put up arms, knowing those left behind will be able to replenish their population. The more you didn't do out of that, the more likely it becomes that the krogan become effecitvely extinct in the fight

-Quarian and Geth tension reaches tipping point and whichever action you go for in ME2 combined with ME3's portion, determines if both coexist peacefully on homeworld, or one wins. If you didn't push for peace in ME2, you don't get to stop the war.
-I would add the Batarians to the fold in ME3, since they have such a distinct identity in the series as the nasty villain race who'd rather kill your wife and children than fight you straight up. Supposedly their government is overbearing and their people are subjected to endless anti-Citadel and human propaganda, so if you dealt with them you'd either have them fighting with you or out of the way. Not sure about the Leviathan of Dis, might just be teaser from BioWare to mess with us.
Just some thoughts