Shepard-"Alright motherfuckers, you all owe me. I'm calling in your tab."
Shepard unites every sentient race into one super-army. Then they use the cannon that took out the Reaper and shoot one down. They research the corpse and reverse engineer weapons to fight the Reapers. Then they have a grand, epic battle and defeat the Reapers. In the end, Shepard sacrifices himself to save the galaxy. But wait, he's not really dead! He escapes just in the nick of time and then he and (insert love interest here) live happily ever after.
-The real question is how will he resolve his love interests from ME1, 2, and 3? I suggest mud fighting, but no one ever takes my suggestions seriously.