the old train man
I do have a ho scale world war two tank im thinking about putting on the layout with a crushed auto behind it.
If you're trying to avoid 'whimsey' (are you?), why not put the tank (some version of the Sherman M4 I guess, those are pretty common models in HO) on a plinth (well, a square representing a concrete pad) as a stuffed and mounted model. Hey, if it worked for old steam locomotives...
Now, as to the other items on your list...
12 footprints -- No muddy areas or swampy land on your layout? Footprints (assuming human...actually, what about deer or horse tracks?). Tire tracks?
13 real weeds growing between the ties of the track -- Real weeds? OK, what kind of micro-bonsai bio-technology are your refusing to use here anyway? OTOH, little used spurs and branches are vegetation magnets, and there is quite an availability of model foliage and grasses nowadays
14 purple houses -- There was one not far from my house, next to the LIRR Montauk line. That said, such colorations for houses (such as pink, purple, turquoise, etc) can be pretty rare if they are not used as retail outlets or art/antique shops...
18 telephone poles with scale working wires -- I presume you mean EZ Line thread here, and while there is definitely a tangle hazzard using it, the results do look nice (and it does snap back under mild deformation).
21 x rated scenes - You're no fun anymore...
For me, it's
No cutsey/overused scenes like cops hiding behind billboards or dogs wizzing on hydrants or firemen rescuing a treed cat (COUGH!woodland scenics mini-scenesCOUGH!)
In conjunction, keep company/business names reasonably realistic (No W.E. Snatchem's here).
No industries with building smaller than the freight cars they recieved will be modeled. Instead, either they will be modeled as flats, with extensions off into the backdrop, or implies via a sizable transload operation I plan to create (with warehousing and unloading equipment - maybe not as cool as branch lines snaking thru canyons between industrial buildings from the pre-Conrail era, but even by the 1980s such old indutrial buildings were being repurposed as offices, public storage, and condos when they weren't being torn down all together.
Oh yes, everything gets a light weathering, but not to George Sellios hyper-depression-era-apocalypse levels.