I didn't get a chance to finish this post, but I started it just after you asked. Personally, I am so done with the Nazi/Wehrmacht default schemes. It seems like that is the first thing people go to, it is like that saying if people talk long enough, "Hitler' will come into the conversation.
As far as themes, that's up to you. Do a German/Nazi/WWII derivative, and you will have initial "Ooh's, and Aah's", then it will fade into obscurity.
How about the "North Korean War", we're still at war with them, that would open all kinds of opportunities. Vietnam would be an interesting era. One of these trampling through the marshes after the Marsh people who tried so hard to hide there. Be interesting politically, as they were hiding from the U.S. and the Vietcong. Jump up to the gulf wars, then you can add some hover capability, and really go for it. Just ideas.