If they did, all the stories would have to be pre-ME3/Reaper invasion or during invasion. Since all the Mass Relays are now destroyed, no inter-galatic travel, while possible, is feasible. Although, perhaps Bioware would create story boards telling us ME fans what actually happen to each of the major races after Shepard's actions/decision on the Citadel!
There story isnt really finished yeah shepard stopped the reaper invasion but thye could do more with them like going to a diffent galaxy or getting the history with the reapers where exactly did the reapers come from all that junk do the reapers do the same cycle in other places there is more they can still bring. Also, maybe they could set it in the future so they rebuild the relays or something. My point is that they go farther out then just the milky way meet even more races expanding mass effect even more.








This is why i don't believe game developers anymore when they say this is the last game of the series.