My personal extended cut?
First: Expand the deal with the Crucible. Have Liara find an ancient recording along with it and slowly decipher bits of it. We learn that the Crucible is not prothean design, but a something they learned from the species that lasted longest before them (which the Prothean guy already says) but this time we learn that it was a system built by the species that became the reapers. This system would be put in place as a way to challenge the cycle of extinction and allow this/these species to put their case before their supreme system (the starchild). Any species that could last long enough to create the crucible would be deemed worthy of that minor "courtesy." However, some species have figured this out, but not in time to build the crucible, and have modified the plans to allow a massively destructive weapon to be built in to it, allowing the option of destroying, or at least critically reducing reaper forces to where military victory is possible.
This would be revealed throughout the game, with the last bit falling into place just before the mission, or being explained by the starchild. Depending on your actions in the game, this could go a number of ways. If you saved the quarians and geth you can point to their budding coexistence and say "screw your idea that organics and synthetics can't coexist, we've got this going on right here and now, and all you can do is threaten to destroy this beginning which could prove a superior end than you ever envisioned." At the same time Harbinger would also be there, arguing for continuing the cycle of destruction. This conversation would veer widely depending on your actions in the game, especially regarding EDI, and the quarians and Geth. This could be a paragon ending where the reapers back off for now to see if you really can bring about peace between organics and synthetics.
If you've gone renegade you could sacrifice earth, pull out the fleets at the last second and blow up all/most of the reapers. This gives time for you to pull out weapons data from the crucible and gives you a fighting chance.
There would be no fusion dance ending because that is a silly thing.
You could also have the join the dark side option, that one would probably remain largely unchanged.
This would resolve my biggest problem with ME3, the crucible and the starchild coming out of nowhere at all and being "hi, we're suddenly important! Look at us!" It makes it more than just building this thing just because.
Blowing up the earth ending would kill shepard and depending on military resources, would have an epilogue of a bloody and costly victory if you have enough. Montage of your crew going out like badasses in the fight against the remaining reapers and husks, which lasts a long time. At the end you have the monolithic human empire created as your legacy. If not enough resources... the reapers manage to be victorious, but are vastly reduced in numbers and are unable to destroy all life in the galaxy. They instead flee to begin the cycle elsewhere.
For the join the reaper side ending, well, that would be a humanoid reaper descending on the fleets of the other races as they futiley fight on.
For the paragon ending you have the reapers leave for a different galaxy to start again, allowing the empires of the galaxy to continue to exist, but with all of them knowing the threat is still out there. This, and fighting the reapers together, creates greater unity between the races, easing the former disparities. The Quarians begin to resettle their world with the help of the Geth (assuming you managed to save both) otherwise there would be focus on EDI and her relationship with the crew as the organic/synthetic relationship. Shepard retires with love interest, everyone rocks out to the credits. Then after the credits, another cinematic set several hundred years later, a crippled reaper drifts in to citadel space where it is discovered that the reapers were annihilated by a superior force from another galaxy. This force now knows about your galaxy and that you let the reapers free to attack them... and the ramifactions will be known in MASS EFFECT 4!