I had an idea and I thought it would be fun... don't know if anyone already suggested this, but...

What if, when playing solo, you get access to "hire" a JTF soldier.

They would be level 1 and if you keep them alive, they level up and get access to use better weapons and give some perks for the player (ammo and armor resupply, for example). If they die, you need to hire other.

Maybe have some utility, like when not in combat they scavenge materials that you pass by. Or you can task them to return to base to sell/destroy (for bonus materials) junk items.

On max level they can revive you once with a cooldown. But I don't expect a huge bonus or great perks that disincentives multiplayer, just enough to be fun and make you care enough for the companion.

I don't know... for me it would be fun. For massive is another opportunity to have cosmetics added. And I think it would be a good thing for people that want to play solo.