Mass Effect 3:
* get rid of mineral-scanning, or at least make it involve some gameplay. Maybe a small trading game would be appropriate; nothing really detailed (like, say, Privateer), but maybe something on par with the drug-trading in Chinatown Wars. In general, drop anything that's too grindy/MMORPG-ish. People will much more readily forgive a few less hours of gameplay than they will boring stuff.
* don't introduce too many new characters. Re-use and further develop old ones, including ones from Mass Effect 1.
* party banter seemed sparse or non-existent in ME2. Bring it back. Better yet, make an option to increase or decrease the frequency of the banter.
* the action side of combat was improved a lot in ME2, and doesn't need a revamp, just incremental improvement, but the stat-crunching/RPG bits, already weak in ME1, took a beating. Beef them up. Add lots of new, interesting abilities and weapon classes, and put a high priority on balancing them properly. (This means Shotguns and Vanguards should be fixed)
* the unique Paragon/Renegade actions (where you had to press a trigger to activate them) were awesome. ME3 should incorporate a lot more of them.
* ME2 retroactively made ME1 better by reading tons of stuff from ME1 saves and following up on that information. Even the little e-mails you'd get from old characters were welcome. Despite the complexity of incorporating history from both ME1 and ME2, ME3 should do it. It's also a nice way of getting new ME3 players more interested in buying the first two games.
* The N7 side missions, where a lot of experimental, relatively half-baked ideas were executed on, were pretty good. Keep doing that sort of thing.
* Drop the summary screen in between missions.