Checking out the screenshots, I don't see the pumpkin behind Rosella. Am I blind? The background doesn't look like Ooga Booga, though. I'd rather see her in a dream version of Tamir, personally, if we get to play her.
If the nature of the other playable characters are just interludes, then I think it's safe to say we'll be Rosella and Alexander in their dream lands. That would be interesting but my worry would be that it would just be short interludes (like the length of Graham's visit to the bizarro Tower Realm.) That would just leave me wanting more.
But the feeling I got from the announcement was more that you could play as other characters who will be journeying alongside Graham. As if once Graham finishes with the Green Isles and goes to Shadrack's Realm (presumably) he won't be the only one going. If that is the case, then I think the other playable characters would be Valanice (because she's up and about) and the Ranger (by default because he seems to know what's up). I guess it could be Cassima or Edgar. But I really think Valanice is one, which just leaves room for one more character, and I can't see including Cassima without Edgar, or Edgar without Cassima. They both have the same stake in this, so bringing one and leaving the other out would be odd.
Oh, and I'm not really seeing the druidic influence in Valanice's outfit. It could be, but I thought the druids were in plainer brown hooded cloaks. Don't know what Valanice is in as it doesn't seem to be her regular dress. I guess it could be a druid cloak and it would make sense and all, but it could just as easily not be.