Consider this a start. As I think of more things I WANT to do, or would LIKE to do, this list may change. The Givens won't change, but the Druthers may. But then, that's the whole point!
Reading Railroad prototype
L-shaped room, see picture for dimensions (link)
30" minimum radius curves
15-20 car trains
Continuous running ability
Allowance for helix to second deck not part of the Reading Railroad
One working yard
Engine servicing facilities for steam and diesel with roundhouse and turntable
Emphasis on freight operation with a few remaining passenger runs
Represent actual locations but does not have to be map accurate, just the 'flavor'
Simulate Crossline traffic on East Penn and Lebanon Valley branches
Code 83 commercial track for mainlines, Code 70 for sidings
#6 turnouts on mainline, #4's for industrial sidings, possibly one 'cosmetic' crossover using #8's