While Mass Effect 3 didn't ruin the entire Mass Effect series or universe, especially not retroactively (Despite the fact that I still felt vitriol towards the ending, I could ignore it, which really, I felt was something of a fault in its own right, but no biggie really, the rest of the game was awesome.), I would be pretty iffy about buying another.
Why? The series is over, at least as far as Commander Shepard is concerned. If they pulled what Halo is pulling now, and did another Shepard game? I'd be wondering why, and if its going to be any good and stay true to what made Mass Effect awesome. If it stays true to the spirit of the series, sure, I might buy it, and I may deeply enjoy it like the other Mass Effects.
Now, if they make a whole new, spin-off series, which is basically their version of Fallout or The Elder Scrolls in space, where you can literally create your own character basically from scratch, choose a race, powers, weapon proficiencies and so on? Oh god yes. Sign me up.
Please Bioware, make this dream a reality. I want to actually play out as a Quarian or Turian, and not just in the decent multiplayer bit of Mass Effect 3. It was fun, but I'm burnt out on it. Give us something similar, but at the same time, different enough to qualify as something new and fresh.