I reckon the "Dark Forest" will only be related to the coaster in general, I can actually see Ug Land keeping the majority of the theming because I really like the prehistoric look, and I doubt that SW6 will look that much out of place amongst it. It's similar to what they did with AIR, Forbidden Valley is a very bizarre, alienated, creepy landscape then you have bright blue elegant Flying Coaster in the background. It looks quite unique to me!
What I'm expecting to happen is that Ug Land will be re-named to "Thunder Rock", (the word thunder emphasizes the fact that part of it is horror related, hence SW6 and the dark forest. Rock on the other hand will be an excuse to keep most of the theming in the area, so Rita doesn't look out of place either. Rita is themed around "Thunder Rock Rally" after all.. Coincedence?)
I doubt they'll do anything to Rita apart from maybe change a bit of the soundtrack to include some eerie sounds and somehow advertise SW6, so that people go on it. If necessary, repainting some of the supports to a grey-ish colour may help Rita fit in with the castle like theming, but all in all, Alton will not change the trains, remove that massive tyre for the entrance and re-theme the queue for a coaster which is still fairly new.
For the name of the ride, I'm expecting Alton to come up with a "name" for this demon, like they named Rita what it is but not as ridiculous. Omen sounds fine but I "can't" see it happening for some reason, and that "Daemn" sounds a bit too corny. I guarantee if we think of some really evil and dark names for a demon, we will not match it up to what Alton would have for a finishing result.