I have always fancied something exactly along the lines of what paul@trinitymusic.karoo.co.uk has suggested. I think I'd be even more 'modest' though (for a living room organ anyway):
Man I & II:
Gedackt 8
Prestant 4 (case)
Wood Flute 4
Spire Quint 2 2/3
Fifteenth 2 (not too narrow)
Ped:
Subbass 16 (from Gedackt, if space)
Gedackt 8 (from flute and gedackt)
I - Ped
Tremulant (maybe)
I believe this could be made to fit in a normal room if the 16' was mitred. There would be folding doors to keep the sound in and the cats out.
Now, if I had a barn to play with ... well, of course there'd be the vintage Ford Triumph Spitfire, the recording studio equipment and the 3-manual console (hooked up to hauptwerk) to fit in, as well as the harpsichord(s) and baby grand, the 4" scale live steam Fairground engine I'd be restoring, the home brewery, oh and a big leather sofa with a very exclusive Hi-fi (not to mention my office with an iMac, and my library). But it would never be complete without one of these delightful organs. They seem to be one of T&B's 'standard models' which they roll out for small chapels and such - after all, the exchange rate is very favourable at the moment. When I die it can go to a worthy recipient, such as a school chapel.
I intend to do my mid-life crisis in style; ideally I'd only enter the main house to eat and sleep (shh, don't tell the wife). When I'm bored I can take my chocolate-brown labradors out for walks on the moors. All I need is a barn and a bit of pocket-money...