in order of personal preference:
1.) a true sequel to mass effect 3 set in the milky way.
i'm ok with mass effect 3 being the end of shepard's story, but i'm too invested in the original setting to get excited about andromeda. there are too many loose ends and unanswered questions left in the milky way for me to be satisfied with where mass effect 3 left things. i'd like to see how things are going post-reapers centuries later. this, however, is arguably the least likely to actually happen considering that bioware would have to take an active interest in un-fucking me3.
2.) remake/remaster of the original trilogy
me2 and me3 hold up pretty well in terms of gameplay, but i would love to see a faithful adaptation of me1 on modern hardware with bioware's current resources. that being said, i should probably be more careful with what i wish for.
3.) a sequel to andromeda
honestly, this is where i start to lose interest. andromeda was a desperate attempt at a fresh start that failed to capture what made the original series so great. could a sequel dramatically improve things? maybe, but my expectations are pretty low at this point.
4.) a first contact war or similar spin-off prequel
while there is definitely a lot of potential for story telling here, i think it would be better suited to a different medium. i don't think a prequel to the original trilogy would fit well within the framework of a mass effect game. it might work as a book or a miniseries, but not an rpg.