World War One would be an awesome setting, and an average joe friendly environment. Weapon types were fairly limited and standardised, the tactics were bloody mindedly simple, and the overall backdrop would be known to all on at least a basic level. 

I don't see how charging an enemy trench enmasse, and then holding it could be seen as any more depressing or dull than snaking through the outskirts of Kabul and combating a terrorist threat with a handful of OMFG special forces. 

Trench warfare was actually quite a colourful business; bleak in that it was nasty to be apart of, but from a narrative perspective it offers loads of interesting scenarios. 

Nothing more grim than bayoneting a 17 year old German youth, having said bayonet get lodged in their rib cage, and then trying to communicate the word "sorry" to them as you pull the trigger to break free. 

If large battles isn't your thing, then they had night patrols didn't they? Where a bunch of guys went beyond the wire at night to do a bit of scouting activity. Read a bit about 'em, and I think they pretty much translated into the special forces missions of tomorrow. 

I dunno, I just think it'd be the easiest one to pull off. 

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On another note, one set during the Warsaw Uprising - now that would be hopelessly depressing. No hope of victory, and hundreds of thousands to die upon the inevitable defeat... maybe a re-imagining of it would be fun. You know, where the Russians [i]didn't[/i] just stand back and watch it all happen so's they could walk in and not deal with any Polish revolutionaries.

EDIT:

Back to the world war one shenanigans quickly,

Lake Naroch Offensive = 500,000 Russians vs less than 50,000 Germans. German victory, of course. Still, I imagine taking part on either side of the wire would make for one Hell of an RP. 

Think what I like most about ww1, is that the soldiers were just people thrown into a horrible situation. There was no Third Reich zeal on the German side, and no FREEDOOM!!!! on the Allied side. It was just loads of dudes, drafted into the army, and chucked at each other so's that the main players of Europe could buy a couple more summer beach houses.