Now that is a difficult question to answer, maybe I'll have a shelf above the other stuff with some plarail track just for the advance trains.
What I have been considering is whether to do a mixed N scale and OO scale layout. What I mean by that is having the OO gauge in the foreground and then some N scale behind it to give a sort of fooled perspective of another line in the distance
The other thing I'm interested to build into the final layout will be multi level tracks for the N scale, although you rarely seem to get OO scale muli-levels due to issues with the trains managing the gradients it seems with Kato N scale its not a problem so I can't see myself sticking to a flat layout for that!
As for where I'm going to build it? Well I had been thinking of using one of my spare bedrooms, but now I'm toying with using my garage instead as I can't fit my car in it (car's too big) so at the moment its only acting as temporary storage for some stuff. Although having said that my car is a bit of a sick puppy at the moment and is in the shop to see if they can find the problem, if not well she may get replaced and if I get a smaller model then the garage is back in use for its original purpose lol!
Boy this year though, it just keeps on giving - crap that is -