if they are gonna do companion system I hope its Trauma Team. imagine helicopter arriving at your location, sniper start shooting enemies, or if you're inside, door is kicked in and guys rushes in with guns blazing etc, or if theres windows, dudes jumps through it and start shooting. Kinda navy-seal-companion system, I havent seen those in game industry before at least. Or a car drives to your location and stops, guy goes behind the door, start shooting, if its on the streets for example. Helicopter for off-road stuff.
They could even use Trauma Team for combat ress. your health drops to zero, you go coma, you see eyes closing, next time your eyes open when some trauma team dude is resurrecting you.
My idea isnt really companion system its "back up" system. Never seen those. Or wait Ubisoft might have one, not sure though.