While I would love to see castle get some of the love that some of the licensed themes have gotten over the years, with occasional huge set releases like the Death Star or Orthanc, I'd rather see a different approach with a modular design. Instead of having one huge set, have smaller sets that could link together in a modular plan. Think, for instance, of some of these classic older sets:
Redo things like that, but at a bigger scale using more modern building techniques (think, for instance, of the Helm's Deep and Uruk Hai Army sets). Sets could be something along these lines:
$50: Central keep
$40: Gatehouse - Gate with two side towers, defenders
$30: Corner tower
$20: Wall section (with hinge)
The wall sections would hook to all of the other sections, and they would also be sort of army builders. The corner towers and central keep could exist as stand-alones, or be hooked up with the walls. Of course there would be special figures with the larger sets, like the royal family with the keep, maybe some special knight as captain of the guard on the gatehouse. The whole idea is that you could buy as much or as little as you want. You could just have a single tower for $30, or you could spend up into the hundreds of dollars by getting a whole series of walls and corner towers, with a gate and a keep. Yes, walls are kind of bland, but by getting several you build up an army. Or they could include siege weapons with the walls, like in some of the classic sets, which would also be an inducement to buy more of them.
The reason I think this might be a good plan is that a lot of people (including me) would probably not get the huge $150 set, but might over time spend more than that building up a castle a little bit at a time. Also for kids, parents are probably going to balk at the big ticket item, but might be happy to get a smaller section, and then add to it over time.