Most veterans were in the service during the M16/full auto days, can't build much of a collection on that basis, unless you're doing handguns only or are very well heeled.
WWII themes are good, Korea as well, that just WWII updated a bit. WWI is good, but guns are scarce and expensive.
One main battle rifle from each of the WWII belligerants would be a good starting point, US, UK, Germany, Italy, Japan, Finland, Russia. Then get into the minor players, like France, Holland, Poland, Hungary, etc., and even into important Neutrals like Turkey, Spain, Switzerland, Sweden. Both sides spent a lot of money and effort keeping the neutrals out of the war or courting them to join it on their side. The Commonwealth had Canada, Australia, and India as major allies, all these had their own rifles. The No. 5 Lee-Enfield was used in the Burma/SE Asia theater exclusively.