I know how late this is considering the Extended Cut came out quite a while ago, but I have recently been replaying all the ME games and I'm on holidays with nothing better to do so this is my idea for a better ending.
In the final battles you can call upon your war assets to help you fight, similar to Dragon Age Origins. You must fight through hordes of Reaper forces which will get progressively harder the closer you get to the beam. The actual run to the beam will stay the same. Everything on the Citadel will stay the same except for the God-Child. Shepard will activate it and pass out/die.
Ending 1. If galactic readiness rating is low then Shepard will activate the Crucible, but it won't work, their plan ruined the cycle continues and everyone dies. It will play Liara's message to future races. It will then flick to Shepard in a bar with Garrus and the crew in the background. Dialogue will vary depending on who/if Shepard romanced anyone.
Ending 2. If galactic readiness rating is medium Shepard will succeed but die and it will flick to Shepard's funeral with Shepard's LI or Admiral Hacket (If no one was romanced or the LI is also dead) giving a speech. After the funeral it will flick to Shepard sitting at a bar with all those that died, Mordin, Thane, possibly Miranda, etc. If Thane was romanced it will show Shepard meting him across the 'sea'.
Ending 3. If galactic readiness is high then Shepard will succeed and pass out. Shepard will wake in a hospital bed and Admiral Hacket will explain that Shepard has been in a unconscious for a few weeks and he tells Shepard what happened while he/she was unconscious. If Shepard romanced someone then their LI will come in to talk. Shepard will have the option to ask to marry them or the LI will ask (Depending on who the LI is), Shepard will have the option to say yes or no but still stay together. It will show one last romance scene after a victory celebration or on Shepard's honeymoon. Like in the Extended Cut, Shepard will say a speech while it shows concept art.