#1 - nothing less then ME3 customization in terms of guns/mods and should be further expanded
#2 - armor stats/mods - non-existant in ME2, the options in ME3 were barebone and made next to no difference. ME1 wasn't satisfying enough in this departement either really. The entire system needs to be expanded
#3 - Mako stats/looks extensive customization. If you're bringing it back, make it feel like it's a natural extension of the "ground" gameplay. Lets us make slower gun-blazing Mako, or a faster platforming-focused version with weak firepower version depending on the playerstyle and how they want to deal with the enemies they encounter (fighting style or running away style). Looks-wise, we should be able to make the vehicle look like our armor
#4 - influence on the game's world depending on playerstyle and what the player invests in. You don't want PC/party members' skills (or lack of them) to lock people from best loot in lockers like in ME1 fine. But if someone's investing in best, most expensive guns, let them blow toughest doors and get better loot. If someone invests in best omni-tools, let them decrypt toughest locks to get better loot.