I have this strange reaction to your post. First, I majored in History in college, and love my historic Lego castle stuff. But I love my fantasy as well.
Fantasy: I'd love even more fantasy than I've got. I wouldn't like to see history pushed out, though. I'd love a big castle set, on the order of the bigger street-corner buildings, as suggested.
On a side note, catapults are a fad, I think. They'll always be a part of Lego castle from now on, but I suspect there will be fewer of them. Dragons, however, are not; they'll be with us in newer forms as time marches on.
Basically, the bigger and more historical Castle I can get, the better. I'm torn. I want both a historical line and a role-playing line with lots more fantasy races and monsters.
About your son: the good news is, although your son may well outgrow his legos and have a Dark Ages, almost all boys and a whole lot of girls love building toys of all varieties; chances are, if you listen to what he likes and wants, he's going to love the entire duplo-through-adult experience.
And, finally, by far the best thing TLG has ever done thus far, in my opinion, is the Medieval Market Village.
-tc