RWBY: Nightmare Grim -Yang & Ruby
@sir-adamus made a great post on how they feel Yang being infected by a Nightmare Grimm would go that can be found here. I broadly agree but here is my overall take for both Yang & Ruby:
Yang:
Yang would basically start, stuck in an arrested development situation, straining to keep a broken family functioning as a small child and her 'glass coffin' would be the cabin in the woods she went to find Raven at.
I think Weiss & Blake would be people who came into the home as well but also need to be cared for. But as the others arrive they begin to age up, fight off aspects of the Grimm & Yang ages with them. This causes a shift in the Grim.
Yang's first stage s basically "Wear her down through attrition of childhood trauma" But when that doesn't work and the others start actively fighting it the Nightmare shifts to "They don't need you anymore and are going to leave you" Cue Yang making the journey to the cabin to be greeted by a Grim-Raven.
Ruby:
Then there's Ruby, I'd say that with V9 implying her modelling herself so heavily after her perfect but also tragically gone mother who is not strictly healthy and she' essentially haunted by someone she didn't know so...
Ruby is all lone wolf huntress, but she's functionally a ghost to the 'public'? In essence she is Summer Rose rather than herself, white cloak and all, and there's likely a decent amount of cemetery and death imagery around.
Ruby's "Give yourself over" event would likely a mysterious mission she needs to go on all alone. One that will take her away from the place she's protecting & the people she cares about there. Her subconscious defense would be needing to be more prepared and dealing with local problems.
Conclusion:
Hilariously, while Yang's nightmare happening wouldn't radically change the plot over much, tons of Ruby's issues would likely come to light sooner if she had a Nightmare infestation and at least scale back her issues going into V9.
