Rita, to me, has always needed something like the Dark Forest to come along and sort of "ground" it into an area. When the new coaster was announced, I was certain of a re-theme of the ride. Yet now I think keeping the Rita as Rita would be a good move.
To the every day person it will look very imposing when they see a ride they can relate to/have ridden before being changed by this other ride. If they changed Rita the whole area would be alien, and the theme of "the forest taking over" would be rather pointless... as its got nothing to take over.
She does not need a new storyline or theme to fit in with the Dark Forest, Rita can be used in a very cunning way instead, it is essentially a massive piece of theming for Thirteen to play off of. Better yet, it's a massive piece of theming thats ridable :)