Just a few ideas to put into the aether, wouldn't really call them suggestions.
Jet Cooler backpack - Allows you to fly 10-25% further. This would be a teardrop legendary only found on the island.
Shield Backpack - gives you extra active shield plates but only for your back (could be helpful for new players).
Decoy grenade - In CS there is a decoy grenade. I envision the arm behaving a little different where it actually spawns 2 holograms to fight eachother. It could make a copy of yourself and an enemy in the area and they start fighting. Might be an interesting combo with mirrors. (the holograms could do actual damage and behave like the bots.)
Resurrection Arm - A rip off of Lifeline from Apex, some kind of variation.
Health Cannon - Like in fortnite with the chug cannon; a support arm to heal teammates.
Testosterone Injection - In GTA they have "Bull Shark Testosterone". It gives you clearer vision, more health and more damage. It could be an arm in this game that gives you more health and ups your fire rate for 15 secs or other.
Boom Box - Fighting around the active DJ booths can be challenging because of the limited audio. An Arm to do the same thing anywhere but it only effects enemies in an affected area.
Cryo Freeze - An arm that completely freezes an enemy for 5-10 secs. The frozen individual is completely impervious to damage while effected but it would turn any 2v1 into a 1v1 for 5-10 secs.
Bee Drone Swarm - Arm that unleashes a swarm of robotic bees on someone. Another idea to turn a 2v1 into a 1v1. Limited damage but very annoying.
Drone Bomb - An arm that releases a drone you control that can fly a certain distance and detonate. GTA has this mechanic. I will note it is super annoying in GTA. I don't like it and it's perfect for the new player.