Personally, I'm fairly pleased with the idea of a new galaxy, there should be lots of things to find -- new species, new galaxies, new governments. We're outsiders, you can imagine how others will feel about those "Milky Way Invaders."
Problem is justification. No race had the ability to travel to other galaxies.
I do suppose there's possibilities.
- Finding a new cache of Prothean data for traveling dark space that was never completed before the Reaper invasion.
- Similar to the above, but rather than Prothean data, it's a data cache from race from the other Galaxy who went to explore Milky Way. That or even their own type of relay that was sitting in the Milky Way.
- Private groups funding dark matter study. There is two years and six months of available time to work with of stuff not happening. The two years between ME1 to ME2. As well as the six months between ME2 to ME3. Perhaps a business is funding N7 to test dark matter energy and colonize new galaxies?
Civilizations were executed every 50,000 years, it's possible that sense both Shepard both them time, two years and six month extra, and even defeated the Reapers, perhaps there was a technological break through.
"Your civilization is based on the technology of the mass relay, our technology. By using it, your society develops along the paths we desire." - Soverign
It could be that the needed 50,000 years was set in stone as efficient time period of advanced enough for harvesting, but not advanced enough to leave the galaxy. Beating that 50,000 allows for further advancement.