Well, in a way we can see them as guest characters, however, the events of DOA and Ninja Gaiden happen in the same universe, so the notion of them being "guest characters" might not be applied in cases like these.
I think most people would rather have original DOA females rather than girls taken from NG. I like having an amazonian female and a caring shrine maiden, but I'd rather have new creations adding more diversity. They are cool so I hope their fighting styles will remain. But I'd have been happier if some of their moves had been given to new females.
They already are original to the DOA cast, so just making a new character with their movesets feels like a unnecessary copy. The new girl with Rachel’s moveset would be a copy of a copy. Unless they die or retire or whatever I don’t think they should be replaced by somebody else.
I just don’t get how they’re being treated as guest by you guys when they’ve been getting the DOA girls treatment ever since they were released. They were featured in the DLC crossover sets unlike guests and they were eligible for DOAX3 character poll. If Rachel was popular in the East and was top 9 she would’ve been in the game.
The arguments for them being DOA girls have always been the same; they are part of the shared universe and get featured in many crossover DLCs. Those arguing against them as DOA girls always seem to disregard this reasoning...
Anyway, I'm saying they should stay at this point. Not priority, but they are quite unique and getting rid of them is stupid. But I prefer to see true original girls like Lisa, Tina, Mila or Christie for variety sake.