I'd like to see lego take castle and fantasy into two separate directions. Redirect castle to be more like medieval market, and take things like trolls, elves, etc and let them populate their own theme. The thing about this would be the purist castle fans would be happy, and while the fantasy theme could still incorporate castle-like elements (towers etc) it would be open to more than just factions and their respective forts.
What I'm thinking of is something like the turn-based strategy video games Heroes of Might and Magic, if anybody here has heard of them. Personally, they took up a lot of my time as a child if for no other reason than the different towns and their creatures. The basic premise is, there's a variety of towns you can build up with castles and buildings that have different attributes, and you use the town to build up an army of creatures that would populate that town and use them to take over other player's stuff.
I really loved these games (specifically HoMM3) because of all the creatures you could have in your ranks. There was the "castle" town that had the basic archers, pikemen, cavaliers, and also angels. Then there was the necromancer town (my favorite) that had the skeletons and zombies and ghosts and liches. Sounds similar to recent LEGO castle lines, right?
among the others (I'll stick to the ones from 3 but there are five full titles and their expansions out now) there was a hellish town that had imps, hell hounds, and devils. A fort town built like a celtic encampment, with wooden buildings and log walls on a grassy hill, that had centaurs, pegasii, dwarves, and tree creatures. Plus a few others. The point is these are the kinds of creatures and sets that could be included in a fantasy lego line. Buildings with odd architecture populated by mythical creatures. Different enough to be its own line, but still able to cross over and work with castle sets nicely if you wanted.
Many towns had dragons, but I'm sure you're all sick of those by now... Anyway, onto the elves. the fifth game had an interesting take on elves, as they had a special race that lived underground that used poisons and whips in their arsenals. They used a lot of purples and greens and the architecture involved a lot of arches and crystal formations. It would be a good enemy for the tree dwelling elves if they need one, or for anybody.
here's a link to a very thorough site about the games I was talking about, it'll show you all the different towns and creatures so you can get an idea what I'm saying.