I think most people would like the style and atmosphere of ME1 but with many, not all, of the refinements of ME2/3 for ME4, with obvious improvements all around. Larger, more diverse environments, no distinction between combat and noncombat areas. More roleplaying and exploration all around with more interesting and useful non-combat approaches to situations.
2/3 were too choreographed. Everything was centered around you specifically, the whole Space Jesus thing got a bit too far, where as in ME1 it mostly just felt like you were there, a small part of a much greater happenings, stumbling along in a galaxy infinitely greater than yourself.
ME2/3 didn't have the sense of wonder and awe that ME1 had, and that wasn't just because they were sequels, it was the tone and direction they took them in. I loved them to death but they were different for sure. BioWare nearly wiped the slate clean with the ME3 ending, so there's a lot of freedom to really change things up for the next installment in the series. They can go back to the ME1 style of wonder and vastness that the world held if they choose to.
I really hope they don't just put the galaxy in a state like little happened in ME3 and everything's back to rainbows and sunshine. I hope they take the most dire of view of the Destruction ending and face the issues head on as to what would really happen if the entire galaxy was almost completely annihilated and the Mass Relay System, the heart and nervous system of their entire society was nearly destroyed.
What kind of discord would ensue, what unlikely alliances would form. How would things transpire if some areas maintained their Mass Relays while others were lost and cut off from the rest of the galaxy for decades. What would Turian worlds cut off from the rest of their society look like, or human colonies, or any of them. What would happen if new forms of travel were developed that allowed for travel without the MRS, what would we find in those vast areas of the galaxy never before available to us. What kind of worlds and societies would we discover. This is the kind of stuff BioWare should be thinking about now for ME4.