Huh, thats easy.
1. Liara, Ashley/Kaidan and Wrex if possible, all coming back as SQUADMATES. Not walk on cameos in supporting roles - SQUADMATES! Liara and Ash/Kai take priority, since those "two" are alive no matter what. Everyone else is potentially dead, and so should come behind them in priority. Also, make characters actually be "characters" again, not bloody "mission objectives". Exploring their personality and their past should be a great optional experience to explore that impacts the plot in a meaningful, UNIQUE way! Not just be some tedious shooter mission to advance the game that ends up being telegraphed to you. "Oh hi Shepard, one of the crew who you hate, think is boring and have ignored until know wants you to solve a very personal problem in the form of an errand that involves killing lots and lots of enemies. From behind cover. With a gun, most likely."
The loyalty mission for Wrex in ME1 may have been the same dull "clean out this prefab" mission which plagued ME1 too much, but the virmire confrontation? Fuck me, how can ME1, a game that ISNT about team loyalty execute an example of it FAR, FAR, FAR better than ME2 ever looked like it could hope of doing.
No, ME2 just said: loyal = normal ability to handle tasks, unloyal = moron who will die attempting tasks. Loyalty should have affected their resolve under certain key moments important to the individual characters, not just their competence in completing simple, straightforward and interchangeable tasks that any number of the other crew could handle!
Anyway, I just want a smaller squad of more well integrated characters this time. A unique role in the story, plenty of banter with Shepard AND THE OTHER SQUADMATES! Things like that. Not just a band of "badass" combat drones like in ME2.
2. A real, focused Bioware story again to end this trilogy. Not another fucking awful collection of busywork sidequests and disposible main plotline which railroads you at seperate moments of the game.
3. Game that doesnt force you into "play me like Gears" ALL of the time. You know, like instead of shoving guns into the players hands all the time, let the powers do more heavy lifting, and not force the player to take cover every firefight and toss their powers like shooting a gun. (Basically: A little more RPG in the combat, less "Gears of War")
4. A gameworld that isnt a collection of small, linear shooter missions, or their associated, small, linear and boring "crossroads" disguised as hubs. I want a REAL hub back like in ME1, where its the centre of the game and actually feels like it. And I want the KOTOR/ME1 style worlds back where they are big events that cant just be tossed aside after an hour of shooting people. They actually MATTER, and have great moments in them that explore the main plot and side characters.
5. More customisation. Armour. Guns. Teammates. Whatever, they all were too thin in ME2. The "upgrade" system was so CRAP and BORING it hurt. Incremental, INVISIBLE stat increases are meaningless. What does it matter when you headshot an enemy with an inconsequential bit of extra damage or ammo? Bring back customising things in a UNIQUE way. The only area I felt ME2 got cusomisation somewhat right was with the modular armour. But it only had 20% of the amount of customisation it should have had. More varied pieces with more unique perks (and drawbacks - stop coddling the player all the time like they are some sort of brain dead, casual gaming pussy).
I shouldnt drivel on any further, as it makes me angry thinking about how ME2 let me down. But there you go, there is my immensely critical, but still constructive to all but the most butthurt fanboy, thoughts on Mass Effect 3 and how it should improve on ME2.
Edit: Forgot!
6. TO DESTROY CERBERUS!
I hate when I have to clarify this, but that anger doesnt come from them being complex, well written and ambiguous "villains". It comes from them being a cliched, "shady" super elite organisation that is just annoying, tedious garbage. Being railroaded into playing as their errand boy in ME2 was one of Biowares lowest ever storytelling points. Rectify that in ME3 by letting us wipe them all out, and decapitate the Illusive Man with our bare hands.