This is discussed
here as well, and there are a lot of opinions on how to go on.
I already posted my opinion on the matter there, so I won't repeat myself that much here, but I think what's most important would be to let the player control more than was allowed in ME3, including opinions and the way to do things.
Another matter which I find annoying in this kind of games is the fact that they are (as someone said in regards to FPS) generally corridors dressed as war. There is no way you can circumvent a certain area, find quicker routes or end up deep in dead ends, simply because its not allowed. Compare this with say Fallout 3, where you can run around in a wide-open sandbox and end up waaay from your intended target if you don't check the map and compass often enough - the borders set up are much less conspicious and kind of make sense: oh, I'm over the mountains, and there we have a... desert. No need for that, let's go back..
This goes for buildings too, but here I think a game like SWAT 4 is a better example: buildings are built like buildings and if there is a door, you can enter it, period.
This goes back to my first point, namely control. To take away control from the player is such a dickish move it should be punishable under international law. ME1 and 2 gave good control to the player: ME3 not so much, especially since so much of the dialogue was automated, and in some instances also slapped opinions on Shepard that the player could not affect. And IIRC, this was one of many issues players had with ME3, so to at least work out that ******, think this, please Bioware:
control.
Control.
CONtrol.
CONTROL.