I see one design thing that's going to give you trouble down the road.

People are going to be clamoring for tactical naval battles.

Shame empire's coming up before it, if total war didn't have naval battles, people would probably not feel nearly as strongly. All honesty, I don't think this setting has enough of an emphasis on naval power to justify the development resources, and I don't think the land battle structure will suit it particularly well. My feeling on the naval strength is that there likely won't be more than one continent on most maps, much like lord of the rings, so the importance of navies will be largely ancilliary, much as how the corsairs of Umbar used them to enhance their mobility rather than to actually fight from them.

I must say the naval battles in empire look very much like the slower, grander big brother of something like pirates! so I'm pretty confident they'll be damn good.

Gamespot currently has the game categorized as  Real-Time Strategy.