Medieval fantasy is based on. It would probably be a good theme, but short lived and pricey. Castle is a timeless theme and always will be, again, it's not a theme that needs an 'in' movie to make it popular. The idea of Lego castle sets was to make your own story up, and even though I would welcome whatever parts a LOTR theme would bring, I'd much prefer more a new range of Lego castle building on what we have. I think Lego castle has been really good these last years, we've pretty much had a lot of what we were crying out for, new human factions, new weapons, new animals, civilian sets etc. It has been good, and I hope to see more of this rather then over priced Liesenced sets. Still, like I said, if castle is put on hold and we get LOTR, I will still get some sets if the parts are good.
Actually, movies provide a pretty good glimpse at what's popular at the time. In the mid-nineties (when the Pirates theme ended) the popularity of pirate movies declined significantly, after a few big-budget pirate movies opened as flops. There were virtually no serious pirate movies between then and Pirates of the Caribbean, which pretty much single-handedly revived the genre. Westerns have been unpopular, at least as far as family films go, for more than a decade. Notice which Lego theme hasn't been revived? The Western theme! Lord of the Rings may not be quite as influential in this respect: high fantasy never underwent quite the same decline in popularity as pirate movies or westerns did; but it certainly didn't hurt the genre and indeed, Lego's fantasy castle line drew heavily from conventions established in Tolkien's work.