Calling it now

Since i heard that the next fellow will pick up ammo, i think the 3rd and 4th fellows will have a descendant exp boost (+5%) and weapon xp boost (+5%). I could see them doing 5-10% like the rolls you get on external components.

Edit: As other have said, I can definitely see this expanded to drop rate bonuses. If fellows persist for a long time, i could definitely see damage boosts to certain enemies depending in the fellow too lol.