I'm interested in 17th century warfare (as you can conclude from my avatar

)! I've got the impression that this era is much neglected by the computer-game-industry

! There seems to be nothing between medieval knights and tricorns.
Let's have the wars between the ottoman empire and europe (jannissaries, akinci, troops of the habsburg military frontier, etc.), or the wars of Louis XIV. I've voted for the 30 YW, as it is the only one available for this era. You can have great atmosphere: units with long hair (a very martial, lion-like look!), deep drums, thunder of guns!
An enhanced siege-combat resoultion would be awesome (sapping and counter-sapping!), and, in combination with the importance of fortifications: a bigger stress on supply-issues. Other than that, I think that the engine (with its focus on the operational level) fits very well. On the contrary, I think it will be very hard if not impossible to adapt the engine to medieval or ancient warfare.
On the other hand, also the 16th century is interesting: for example take the Huguenot Wars. I can image my "reiters" caracoling to the sound of 16th century music (Monteverdi!)!

Or what about the 80 years war? Or what about Lepanto, let's have a big sea-battle! Oh....the spanish armada?
Antiquity? WW II? Knights? Come on, there are plenty of those games out there!