Haute Couture Princesses/Heroines/Villains?    I'm down!!
but I don't think Disney would allow it 

     On the whole the Designer Princess collection was safe and modest.  Realistically I don't see the characters being portrayed this way, even though I think it could be fresh and awesome.   I was a little surprised they took such liberties with the villains- some of them (ahem-Cruella) were pretty racy.    So I  might be mistaken, but they'll probably go for a different approach.   Love the concept though!
I would want princes ONLY if they do them justice and don't get lazy.
I think Heroines or Fairies would look cool, but there might be concern about their marketability, as others have said.   I do think Merida would be included if there were to be another Designer collection.   
And I know it wouldn't happen, but I'd love to see something of Lotte and/or Tiger Lily, two of my fav supporting ladies.
What if they're leaving the Designer collections behind entirely?   Maybe starting with something fresh?