Ability to boost teammates over walls/into vents/etc.
I think an interesting addition to the game would be the ability for one teammate to be able to crouch and have another teammate stand on him for the sake of extra manuverability. This would add an extra layer of depth to map navigation and encourage players to stick together in a group for longer. An example of this mechanic can be seen in this clip from another game.
https://www.youtube.com/watch?v=AKnVbuytnSI