Well my 2c worth,
I would like to see in ME3 the option for 2 styles of play,
basic: the ME2 way of doing things
Advanced: the ME1 way of doing things
don't get me wrong ME2 is a great game, however, I liked the elevator rides it was better than a loading screen any day of the week, I loved driving around in the mako! it was like proper prospecting with a tank to boot.
I loved having multiple armours and weapons that could be customised the way you want, the 10 levels of tech for each item, I liked the openess of the universe and the multitiude of side missions. I will say I like the way ME2 opens up the Story and the universe better and a more controlled way of activating side missions would be an enhancement to the open universe.
I would like to see a return to the character development that was available in ME1, ME2 is just too Dumbed down by comparison.
The final thing I would like to see is a return of the "View Bob" the gun sites and the running is too mechanical in ME2 I liked having to compensate for sway on my sniper rifle and the feeling of movement that was there in ME1.
Oh and one last, last thing, the film grain on ME2 was greatly reduced and I think I preferred the Film grain effect and motion blur in ME1 more,
Saying that the ME2 world environments were way superior to ME1 so that obviously would be the way to go.
PS, give us back the Presidium!