I'm kind of seeing a split into two principal elements:
In game placement enhanced editor
- easy repetition for walls/fences etc
- colouring, shading, texturing including import feature for custom textures) of modules/elements
Spin out of game (like Character Creator)
- using modular approach to construct buildings
(though it could actually be used to create other inanimate objects (remember that funky helicopter?))
- ability to save the new composition so it could be used in multiple scenes/games
(avoid having to try recreate - re-usability is key)
- ability to take a saved composition and re-imagine it so optimise but retain elemnt list so it could be broken apart again
And whilst not critical initially, probably needs to be 'designed on' from the beginning - destructible elements. Maybe all it needs for now is a flag 'set up' set to 0 and not used for now, but at a later stage be able to invoke it.
My 2P worth...? :-|