Shadow has a tri-faction and very interesting moveset. I think players should be able to use those.
My idea is that after finishing the story, we can unlock this moveset. It will only work if
1) player has absolutely nothing equipped.
2) We will get a shield like shadow gets but it will act like a 1 hit protection shield. It can have a 10 sec cooldown.
3) We can also use the shadow moves that shadow use.
The main theme is that we would be extremely vulnerable since we will have basically no defence and no ranged option. We won't have any perks since no equipment. All we will have are some cool unarmed move and a few shadow moves. This won't break the game in any way. Infact, it can be used to do some challenge fights with it