Holy Limited Imagination.
On hundred Billion (100,000,000,000) unexplored stars in the Milky Way, never mind how many planets that number of systems may contain, not enough potential?
The sad fact is that there is obviously more than enough real estate left in the Milky Way for any number of follow on s, it's just that the terrible writing decisions of ME3, compounded by the ensuing decision to not canonize any one ending or use some other plot device like indoctrination, left them with no plausible setting, excepting an entirely new galaxy.
Exceptionally sad in that they had a pre-packaged course all set up with a destroy or refuse ending leading to a new Milky Way cycle containing Liara's Beacons and all the information in them on this cycles' successes and failures, and how to defeat the Reapers. That is just one out of many possible courses that could have left the franchise in the largely unexplored MW, with new characters, and still containing all the fan favorite storylines they spent a decade developing.
Bad writing and strategic vision do not equal lack of potential in an entire galaxy.
Canonization of the endings would double down on the stupid. It would never end the horror of the ME3 ending - there is no way to move beyond it in your scenario, except now we're stuck with a dead setting still.
No, there isn't any potential. Those systems are either exterminated husks devoid of all life because they were part of the Reaper genocide of cycles past, or they are modern level or less advanced societies because they did not advance to the level acceptable to be part of galactic civilization.
That's it. There's no opportunity for any of those planets to escape the most basic premise of the MW as a setting: that the Reapers eradicated all life. The only option is to have a super hidden away even OLDER precursor society that somehow went extinct before the Reapers and no one found it, or invaders from another galaxy, but the MW is dead.
The last option is to basically have a parallel society that evolved around the same time as the Citadel, but we know that's basically nonsense because Citadel space is huge. A parallel society developing for 50,000 years that no one happened to find until AFTER ME3 is more contrived than anything MEA does.