There are some models you never can get enough variety of:
lamps (indoor + outdoor, spot lights, ceiling lights, street lamps, torches, ...)
Depending on the map also:
weapon crates (open)
fences and railings
tools like a hammer, nails, broom, shovel, pick, wheelbarrow
maybe even some dirt/debris piles like a pile of stones, bricks, metal, ...
And if still don't know what to do, try some crystals/minerals/rocks/gems. I still need millions of them for exhibition
I'm just thinking, some modular windows and doors that you can easily fit into/onto any wall would be awesome. Or a set of modular railway tracks and modular street parts. Or modular wall skirtings to quickly add some detail to empty rooms.