I second a Spain vs Netherlands pairing for the photo shoot! Just as soon as I get a wig for it, my Holland will be 100% complete and ready to go.
As for other pairings/groups? I can think of a couple, but admittedly they're excuses to find places for the DPRK to 'fit in' (as if North Korea ever could). Anyway, here goes:
Asian Countries: China, Japan, Taiwan, the Koreas
North & South Korea (Korean War? Don't know if we have an SK)
Communist (or former communist) States: Russia, Belarus, Poland, Hungary, China, NK, Baltic states
And some that aren't trying to find a home for DPRK:
Micronations: Sealand, Liechtenstein, Monaco and any other tiny countries represented.
Entente (WWI): England, France, Russia, Japan, America, Canada, Belgium
Central Powers (WWI, again!): Germany & Prussia, Austria, Hungary, Turkey
That's all I can think of for the moment..