I'd love to see a game where you could play as a Warlord in Genghis Kahn's army, or even just a common soldier. I think it would make a great action RPG or maybe an RTS/RPG.
World War 1 could be a great setting if done right. It would have to have great dialog and excellent atmosphere but it could be done. I don't think making it combat-centric like the Call of Duty series, as the quickest firing weapon you could have would be the Lee-Enfield (a fine weapon) but no SMGs, Machine guns required a crew of at least 5 to use properly and grenades were pretty damn primitive. They could do a good job with full on battles like Verdun, the Somme River, but they'd have to be really well designed. and maybe some of the really early/late battles where they had some mobility. But overall I think it would be a great psychological thriller, if they could really make the player experience the horrors of WWI, living in the trenches, watching you friends butchered in full on assaults against. Brutal bombardments that lasted for days at a time.
Another one I wouldn't mind seeing is something based in the Classical period that didn't involve mythology. Something in ancient Greece/Rome or maybe Egypt.