So let me make myself clear again. The ending of ME3 is bad. Really bad. For those who defend it I believe you have no taste in good media. That being said, no matter which ending you chose something very interesting happens, all those alien fleets are stranded in Sol with soldiers numbering in the tens of millions. They all have no choice but to live on Earth until they can invent a way to travel back home. Imagine then if DLC was set hundreds of years after the defeat of the Reapers. What's life like now on Earth? How have the other races adapted? How do the humans treat the offspring of their saviors, or in the case of the Asari and Krogan, how do they treat their living saviors? There is so much intrigue that comes from that I'd be curious to see what Bioware does with that? Thoughts.

The Mass Relays aren't the only way to travel through space. They still have FTL engines in their ships that can travel multiple (several hundreds or thousands I think) times the speed of light. So it'll be a few year's journey, but everyone in the fleet, assuming they survived, should be able to find their way home. They're not all stuck in our solar system. 
 
Despite that, yeah, I'd love to see some of the consequences the ending has on the universe and the relations between all the races in the long term. With travel between systems taking years instead of literally no time at all. And with the citadel in Earth orbit. Resources are gonna be a lot harder to get to and it'll be a tougher universe to live in. Also, galactic populations is reduced in size enormously, with the Reapers having slaughtered billions.

@RobinOttens: You're underestimating how big Space actually is. Even traveling several times faster than the speed of light it would still put them at a journeys lasting a couple thousand years. Also if you read the codex it mentions how invaluable the Mass Relays are to travel throughout the galaxy. No relays and everyone's stuck in their given systems so to speak.

I know how big space is, really. :)   The Mass Relays were invaluable because they made travel time non-existent, but it was established that most races have developed FTL as well, to travel between planets and within star clusters.  Those quantum entanglement communication things are more prevalent now apparently, so they don't need the mass relays for interstellar communication anymore either. 
 
Our galaxy is about 100000 lightyears across. I think somewhere in the games it's mentioned they can travel at least 12 light years in a day, I believe it was more than that even. So 4380 lightyears in a year. About 25 years to travel to the other side of the galaxy. That's a big undertaking, but do-able. So I'm guessing the next 20 years after the end of ME3 everyone's gonna be traveling home. That by itself could make a really cool game or DLC. 
 
But any DLC Bioware puts out is probably going to take place before or during the final mission, given the nature of the post-game save.