With all the classes that were exclusive to multiplayer, I want more classes, with more powers and variations. ME3 saw the return of an in depth skill tree system. It was a lot more advanced and evolved than ME2's, without all the redundancy of ME1's leveling. It even allowed each class to be played in different ways. I could play an Engineer who is a drone lord, or I could play as a red mage combining elemental "magic" with gun fire. Also all the powers looked and behaved unique, unlike in ME1 where all powers looked the same, or ME2 where they only varied slightly upon leveling them.
In fact, ME1 is a very redundant game when it comes to its mechanics. How often did you find yourself changing your guns or your armor? Not a lot when you consider how you can quickly be drowned in loot, a lot of it which was useless except for getting the rich achievement. ME1's powers had different effects on enemies, but they all looked the same. Same goes for the guns, each class of weapon fired the same. ME3 was swimming in guns, and all of them had different ways to use them, and with the weight system you could mix and match to suit your play style. I say take ME3's gameplay, evolve it further, and actually make the game hard. ME2 had the right idea allowing our enemies to take on their own classes. You would find tech armor guards, vanguards, drone summoners... their AI just sucked. I'd love to be cornered by 3 vanguards with the threat of them biotic charging me. Kind of like how fighting the Citadel DLC boss worked. Best boss fight in the series.
As for customization, instead of limiting our color pallet, allow us to use those spectrum squares you see in photo programs. I would have loved a purple hair option for a male Shepard so I could fulfill my Buzz Lightyear role play. Add that same system to armors.