I'd be content if we could cover over existing windows with any of the fake ones. The gothic stained glass ones cover existing windows just fine, but they only fit a certain style of decorating.

An option to have a completely blank interior as an alternative to the default would be very nice though. Then Square, please go one step further and let us choose whichever interior style we want.
I really don't see why a Lavender Beds interior can't be used in the Goblet for example. With customised exteriors, it makes little sense to keep the interiors strictly region locked.