@CptData
Sounds like Bioware would have to write three different 'games' altogether. Though do bear in mind that according the Codex 'less than 1%' of the Milky Way had been surveyed or explored in ME1.
though Bioware has contradicted the codex before though...
Not really. Main differences?
Destruction: Shepard may have / have not survived the end of the war. The Reapers are gone.
Cure / no cure: either the Krogans have their own realm with a couple of systems and Wrex/Wreav in charge, or they're still fighting for survival in their home system with no hope for a cure. They're still everywhere in the universe and a lot of them are still mercenaries.
Quarians: if helped, they're still there. If not, only a few small colonies survived that war, and they do no longer have a fleet.
Geth: not gone, thanks to some surviving Geth post-war in a remote system not touched by the Reapers or the de-reaperize-shockwave. They helped to reboot the other Geth.
Synthesis: Shepard's dead. Reapers returned to the core of the galaxy and work as some kind of guardians. Not much of 'em is heard so far.
Cure / no cure: Krogans are cured always, thanks to the synthesis magic.
Quarians: if Shepard decided for 'em, they rebuilt the Geth. if Shepard decided against them, the few survivors are incorporated into the new Geth realm and full citizens as well.
Geth: either got rebuilt or survived the war.
Control: Identical to Destruction in most cases, except Shepard's fate (always new avatar of the Reapers) and the Reapers returned to the core of the galaxy, no longer interacting with the galaxy.
The BIGGEST difference in my eyes is the texture!mod for synthesis ending and how to explain the whereabouts of all those races influenced by Shepard's decision. Since nearly no decision is absolutely killing a species (except the Rachni), the Geth/Quarians/Krogans are still around in the next game even if Shepard decided against them.