These are some great ideas, unfortunately the AAA developers don't like to take chances. WW2 sells, modern warfare sells, space marines sell, and medieval fantasy sells. I can't believe that a game like Assassin's Creed was ever made. Why not just set in in WW2 and sell a million more copies?
The setting I'd like to see - the future. Not the near future Halo/Mass Effect where humanity is pretty much the same as it is now, but the distant, distant future. Where space ships are obsolete because people can teleport anywhere they like. Where genetic manipulation and body modifications are something you do in your bathroom. Humans have coordinated thousands of planets, and aliens exist, but are hundreds of thousands of light-years away. I'm thinking an adventure game with a protagonist from 20th century Earth and half of it is just understanding the limitless technology of the age and being amazed.
Also, the Civilizations games allow you to play through all of human history so far, although it is turn-based strategy. And the Warband games takes place in an imagined medieval England/Mongolia/Middle East/Ireland/Scandinavia-land and has some of best first person, realistic fighting of that era, not to mention the best mounted combat of any game ever made.