This type of pack is long awaited and will come in handy. Hope you include a lot in this pack! There's a ton of things I can think of that would be good to include but in my mind, structure wise, these are the essentials:
-Small, Medium, and Large generic buildings that could be used as office space, shops, restaurants, whatever.
-A nice variation of houses, anything from small single story old style houses (60's and 70's pattern housing), to multiple story modern houses, some apartment buildings, duplex's and such.
-A bunch of generic stores and businesses, easily recognizable to what they are, e.g. big box stores (walmart, k-mart, etc), fast food restaurants, convenience stores, grocery stores, and strip mall type storefronts
-CLUTTER! Streetlights (perhaps a lofted variation as well), benches, mailboxes, small decorative trees, parking meters, garbage bins, etc, all that common stuff you see on the side of a road or in people's lawns.
- Cars, if possible, new auto models, cause unless you want to use ones from specific routes, there ain't a whole lot of US specific automotive power, and furthermore, as static objects, the only ones you can "park" without lights turning on at night are the 4 north american models (Tractor-Trailer, family car, pickup, and truck01 or whatever it os), and this may be a stretch, but if you could incorporate it with a vehicle path script, it would be great to see variety without having to see the default-non-North American cars
-I know you mentioned we all need bridges, but seeing as how true the statement is, a few girder bridges would be great! we have a few great models already, but it just looks un-natural when all the bridges are identical!
to your list of questions i'd personally say:
-basics: mostly what i covered above
-time periods: seeing as in our modern day in age we still see structures from the early 20th century here and there, cover the modern era, which conveniently includes buldings from all throughout the past 80-90 decades (with a few really old things here and there
-I'd say each model itself should have perhaps one to two variations, but mostly just have a good, large number of different structures, like for a gas station, don't just use the same model with different colors, mess with the arrangement, move the pumps around or make the convenience store different or heck, throw in a car wash along the back, things like that near the track are what make scenery stand out, and hopefully, using simple shapes can keep the poly count down but still look good.
-small town or big city: both, include buildings that can fit in within the confines of either. We need a few skyscrapers, but we need a few small town staples just as much, so i'd say focus on buildings that can fit in a mid sized city or town and have a smaller number of location determined buildings.
-I know this is impossible and cliche, but both! both are needed, I'd say focus on buildings that incorporate clutter if that makes sense.
-Easy on the foliage, but don't leave it out, smaller plants are necessary, but due to the current variety of buildings, or lack thereof, i'd say buildings should take priority.
That's just my
! I know whatever you end up doing will be fantastic! this is just my little wishlist. Best of luck with the project!