While the image of WWI often relates to muddy trenches, illness, and crossing no man's land, there really was a great deal more to it than that. Personally, I could see a great deal of fun in an RTS of this aspect (did you know that mining was a popular method in WWI, and the allies successfully sunk thousands of Germans in an instant by bombing underneath their trench once?). But truly only the Western front experienced such stagnation. The Eastern front was much more fluid, asymmetrical warfare took place in the Ottoman Empire, U-boat warfare covered the Mediterranean and Pacific, mountain warfare in Italy, fluidity in the Balkans...it really was more exciting than Hollywood has let on. I also think a broad strategy game covering Europe and the Middle East could be rather interesting, even with difficult-to-break stalemates in some regions.