Paragon ending: the Reapers succeed in entering the Milky Way, but choose to spare the galaxy because Shepard has demonstrated herself to be a superior lifeform.
I can't see that happening.
Renegade ending: as a last resort, Shepard prevents the Reapers from returning by destroying every mass relay, thus sparing all sentient life at the cost of intergalactic communication.
They are deep out in the middle of, literally, nowhere in space, the only relay they could have used was the citadel and that was not activated. At the end of ME2 you see the reaper fleet engaging FTL drives and heading for the galaxy.
Alternatively, Paragon: "Gondor calls for aid" ending where the Quarians lead by Tali, the Rachni, the Krogan lead by Wrex, the Omega forces under Aria, the mercenaries lead by Zaeed, Cerberus, the Council races by Anderson, and the Shadow Broker organization under Liara unite all under Shepard to repel the Reaper invasion.
I don't know how much Omega or Zaeed will help, (though I do hope Zaeed shows up, I liked him), but yeah Gondor Calls for Aid is a likely ending.
Renegade: Cerberus, using the Collector base technology, succeeds in turning Shepard into a Reaper, who then saves mankind by negotiating with the Reaper armada at the expense of the other races.
I don't think Shepherd is going to be a reaper, but the renegade option is to use the reaper technology, which means future tech will continue to be based off reaper tech and as such easier for the reaper's to counter than entirely new tech, and possibly cerberus's own minireaper.
Alternatively-alternatively, the beings that made the Reapers did so with the intent of eventually see a species evolve with an individual capable of defeating their creations. The superbeings then intervene and annihilate the Reapers, and offer Shepard to become the new Bhaal -- er, Guardian of the Universe. Paragon declines, renegade accepts.
Maybe, though I suspect the reapers already eliminated their original creators (or they became them).
The Illusive Man is a cyborg created by, or working for, Councilor Udina.
Nope, he might have implants but he was indeed human (Mass Effect Ascension, the 2nd novel, gives a very brief background on him).
Shepard was a Reaper the whole time.
I lol'd.
The Turian councilor was partially indoctrinated the whole time.
Racist!
A Hanar party member that can't use weapons, but has extremely powerful biotics.
That would be awesome.
The Reapers revive an indoctrinated Kaidan or Ashley in a last-ditch effort to emotionally break Shepard. Or, all of the Normandy-S1's crew in addition to every teammate that died in ME2.
Interesting, but I think most of the corpses are safely destroyed.
If no romances were pursued throughout the three games, Joker becomes a romance option for female Shepard.
That would be hilarious and awesome.
I'm aware most of these are unlikely to happen. What I'm actually expecting is some ending where Shepard uses Prothean advancements to turn the mass relays into cannons in order to annihilate the Reapers or something like that.
That would be a good idea, though that probably wouldn't be prothean technology (they barely had just finished their own small relay), or at least not entierly. There are a LOT of reapers though, over 300 in the end cutscene alone.