The social and political landscape of 100 years in the future ought to be pretty different, too.
Idea 4 does cover a lot of bases. It's set in a time frame where people who want anti-human bigotry and humans to still be scrappy 'dude, where's my respect?' underdogs can have that, people who want to play Geth can play Geth without having to be NuGeth, none of the questions or implications that come up from the endings need to be dealt with, and it can still feel like Mass Effect since it's during the time the games cover.
That said, if it's really that popular to be after the end, why not 10 years? It's long enough for the lion's share of resources to have gone into reconstruction rather than innovation, and for Admiral Crazy to make some NuGeth deliberately designed to achieve sentience (which raises its own issues because the Geth becoming AI was an accident and the illegality of creating AI is probably something that would still be on the books even if there isn't much in the way of resources to enforce it, and even if people could be reasoned with to admit that the Geth deserve it for their hand in helping beat the Reapers, it's resources that could've gone into something else, no matter how beneficial NuGeth
slave labour would be).