If I had a choice, I'd really rather Jennifer from Labyrinth, I think those rights are included, maybe.....
Tinkerbell would be a contender, and no doubt has to be done at some point in time, tho I don't think first. As to size, I'd say something around the size of the Frodo PF, that way she is still smaller, as she should be, but that's large enough to still get the detail.
Snow White would be a good choice, since she is very well know and you get that matching set with the Evil Queen, but yeah, maybe not as popular.
If we are more talking about 'kids' here, Then I think either Ariel or Belle would be the big draw card and not just for kids, I really want those too.
However, if we broaden the 'princess' turn just a little, then the first up, sure fire sale killer that isn't a villain or live action, is easy....
Jessica Rabbit, make an Evil Queen quality, drop dead beautiful, hot and dare I say, 'sexy' PF of her, with a cool EX and watch her sell lightning fast, with very few cancellations later on.
I so want a Belle PF and matching Beast, it was at the very top of my list the day the Disney PF line was announced. I want those just about above anything else that I even already have.
As for general look, I've thought about that as well and while I'd take anything that looks fantastic, my preference would be:
Belle: the dress that she wears at the start of the film, but with the cloak she has when she visits the castle for the first time to find her father. The Exclusive could be Mrs Potts and chip.
Beast: the costume from around when he shows her the library, again it has a cloak, but also covers up most of the 'fur', apart from the head, hands and feet. Making that easier to do and more believable.
Go all out on the Exclusive and include Cogsworth and Lumiere.
Basically, you end up with more or less:
Then, as a extra special purchase option, make a matching scale table, with the rose under cover.
However, more then likely, if or WHEN they get made, the ballroom costumes are the most likely, which is fine for the Beast, tho getting Belle's dress looking right will be a serious challenge, even more so when it comes to production and that pesky Quality Control.