I guess if I had to pick a Theme, it would be International! Here's why:
I build only 1/72 scale Aircraft, 1/72 and 1/76 scale Armor, and my ships range mostly from 1/450 to 1/700 scale. I group them by nation, except for my Middle Eastern aircraft. They are all grouped together.
I have aircraft from the following Air Forces:
Britain, US, Canada, Australia, Germany, Russia, France, Italy, Japan, Israel, India,
and about 9 different Arab countries.
I have Armor, artillery and soft-skinned vehicles from:
US, Britain, Germany, Italy, Canada and Australia, Japan, Russia, Israel, Egypt, and Iraq.
I have Navy ships, and some commercial vessels from:
US, Britain, Germany, France, Russia and Japan, with a couple Australian ships as well.
I also have Helicopters only in 1/72 scale. I have helicopters from the US, Britain, Germany, France, Russia, Japan, Israel, Iraq, and Iran.
Aside from all that, I have the following for variety: two cars: James Bond's Astron Martin DB5, (with figures) and Tom Daniel's Red Baron, as well as some HO scale buildings (about 5)