WW1 wouldn't make a good conflict for a First person shooter, the war was characterised by artillery bombardments, machine guns,and charges of 60,000 men resulting in only feet of territory gained and incredebly large losses, and odds were that that loss would be you. Prehaps the WW1 conflicts in East Africa or Iraq would be more fast moving but still as a conflict it was generall sitting in a hole waiting to die.
Falklands would be an interresting conflict to make a game about, it was short but there's plenty of battles to reneact, Goose Green etc.
I don't think the napoleonic wars would be very good either, Large blocks of men walking in formation, it harks more of a RTS or TBS game than a FPS. Maybe some of the rebellions in the peninsular war would be interresting but i don't think there's enought here.
I think a made up future war would be good, and i like the idea of some coldwar turned hot world war set in the 60's would be the most interresting. The British Fighting the Chinese and Communist Arab states to secure its colonies in the far east and africa/middle east. The Americans prehaps fighting a communist bloc of latin american countries and helping a European force in Western Europe fighting against the USSR.
I think the FPS needs more imagination in its themes etc. Setting a war in Madeupistan against terrorists or nationalist communist russians is hardly original. I'd rather be fighting the Chinese Army in Northern India or the Warsaw Pact in Holland.