I'd like it if you had to do different playstyles based on which class you picked. In ME1, you couldn't always go guns blazing, especially on higher difficulties with classes like the Adept, Engineer, and Sentinel, so that required you to think more tactically. In ME2 and ME3, you could do really well with any class without much difficulty, and the only differences between the classes were their available weapons (ME2) and their available abilities. ME3 made it worse by making it so that every class could use every kind of weapon.
While I did like the loadout flexibility in ME3, it made it less tactical and more of a 'shoot-em-up' game, imo. None of the classes had disadvantages that couldn't be covered by firepower, so it removed quite a bit of the difficulty from the game. I'd like for BW to go back to how the class differences were in ME1, making it so that their disadvantages can cause you to have to think of better strategies than 'shoot everything'.