Mass Effect does work well with a 3 player team. As soon as you get 4 player teams, it'll be less likely that hallway through the game you would choose anyone but your already picked 4 team as you'll be able to pick every strong ability for any encounter.
If they rebalance the game, they shouldn't increase or decrease any abilities/powers/strengths etc. They should maybe think of things like In a team These 2 team members together would make team member X and Y cancel each others powers/buffs/debuffs etc, but it will increase X,Y and Z. It will make for more tactical decisions, however, just keeping it at 3 would be a lot easier and make the game feel and play just like every other ME, don't fix something that's not broken...