My basic idea is as such: Humanity goes Paragon and loses most of its Renegades (not that there aren't still evil humans, but we're talking about mainstream culture here, nor should Renegades be conflated with evil) due to isolation. The other two Paragon fics went with the more structured route: if Neutral Humanity encountered and fought the Turians and Renegade Humanity encountered and was nearly exterminated by the Batarians, Paragon Humanity should logically encounter and be at peace with the Asari. My problem with this is twofold: it doesn't provide much reason for Human culture to go Paragon and lose its Renegade side, and it makes a strange kind of alignment-mapping of the other species that doesn't fit. The Asari have e-democracy and are peaceful, sure, but just running into them doesn't mean that Humanity is going to end up like that, and there are plenty of bad things about the Asari too: Illium essentially has legalized slavery and is an Asari world in all but name, and a culture with that long-lived of members is going to pick up some serious conservatism.
So, my thought is that the same Unseen Fate that lead Renegade Humanity to meet the Hegemony will inspire a Prothean craft to attempt a suicide jump into the Earthward side of Relay 314 for Reaper-killing glory. It doesn't work out (Relays are made of tough stuff, the Alpha Relay was so pumped full of energy by comparison that just about anything would have set it off), and Relay 314 is badly damaged but not destroyed in full. As a consequence, Humanity is contained behind it for a century or so, unable to expand further into the Relay Network and without reason or technology to make the long journey. In time, enough is learned about the Relay to return it to functionality, and by then Humanity has flourished into a grand and advanced multi-stellar civilization that had no reason whatsoever to avoid developing technology that would thoroughly violate the Citadel Conventions. Things get somewhat awkward when the human ship's AI tries to contact the Turian ship's assumed AI and gets some very upset aliens in response.