Keeps could easily be done.
Castles... I'd want them restricted to their existing footprint, which, of course, makes it a bit hard to do much by the way of customization. Still, I guess whatever people wanted to do. It'd open the aspect of building ruined castles, et cetera. Limit them to the same # of floors as they are presently, and all's good.
I think the L-shapes could use a customization too.
But the trouble with these is EA doesn't know how to set it up so that you can't build outside of the footprint, so only squares work.
They ALSO need to figure out how to allow your roof tiles to be placed like other houses without having to sacrifice inner squares to do it.
Basically the housing tool needs a revisit. But I don't see it happening in UO's lifetime.