An ending (well, overall plot) that I could see happening:
The Reapers concentrate on the human race first, deciding the that species that took down one of their own has the most potential. As the vast bulk of the Reaper Armada surrounds Earth (you need a LOT of ships to effectively blockade a planet) in order to harvest the highest number of humans possible in the shortest amount of time, Shepard learns that the sun in the Dholen system (the system where you recruit Tali in ME2) was burning out due to the Geth testing a doomsday weapon, and that they've perfected the weapon so that it can cause an instant supernova.
The Council, discovers this, and decides to cause the Earth's sun to go supernova in order to destroy the bulk of the Reapers in one blow, infuriating the Illusive Man, both because they're throwing humanity to the curb to save their own skins, and because their inaction prevented even the possibility of an organized defense, despite Shepard, Anderson and Cerberus repeatedly warning them. Shepard then has to race against both agents of the Reapers AND the Council (including SpecTRes) to acquire the doomsday devices from the Geth, since the Council won't trust a human SpecTRe associated with Cerberus to go along with their plan.
Once acquired, Shepard then has to decide to either sacrifice Earth as the Council planned, in order to end the war in a single stroke but sacrificing the bulk of humanity, or threaten the homeworlds of the Council races in order to coerce them into sending their fleets to confront the Reapers. If he chooses to sacrifice Earth, he loses the loyalty of the Cerberus operatives (and has to fight them). If he chooses to threaten the Council worlds, he loses any teammates from Council races and has to fight them.
In either event, a key part of the strategy against the Reapers will be the discovery that they possess a Mass Effect device that prevents ships from using FTL drives. Shepard and his remaining teammates get aboard a Reaper and sabotage it so that it prevents FTL drives from functioning (preventing the Reapers from scattering and regrouping later).
If Earth is sacrificed, the rest of the Galaxy is largely unscathed, but humanity's fate will depend largely on choices made during the game (for example, the Turian/Salarian/Asari Councilors might try to force humanity off the Council due to their weakened state, but the Geth could then show up with an armada and proclaim that humans are their buddies, and Geth don't LIKE it when you push their friends around ...).
If Earth is saved, the Galaxy is devastated by the war, and everybody has to try and rebuild, and the balance of power is changed in the Galaxy, depending on choices made during the game.