Tactics have basically been absent from the Age of series so far. Yeah, there has been the extended rock/paper/scissors business but that's about as primitive as tactics can be. That was ok for the the first two games and AoM, but it doesn't really hold water in the colonial period. As far as I can tell from movies, books, and common sense, battle during and prior to the middle ages was pretty mixed up.
But as gunpowder takes over, battle becomes increasingly formation-driven. (Not formations-in-AoK, which are basically useless except for dissipating onager fire or whatnot).
So any decent game focusing on this period should include a lot of options - flanking, retreating, charging, etc. Stuff that you would expect from a wargame.
One of the best tactical games that I've played is Sid Meier's Gettysburg, which came out maybe 4 years ago. You can find it at pretty much any computer store for under $10 and I would recommend checking it out. It handles leadership, morale, experience, battle fatigue, formations, flanks, unit organization and so forth, all in a real-time setting. (Rather like Rome:Total War, but the graphics are really bad, and the battle system stricter).
To take the whole tactics-as-a-requirement idea to an extreme: can you imagine World War I included in a game without trench warfare? It wouldn't have the slightest resemblance to WWI.
I'm wondering how these features could be implemented in an RTS game. Which features could be included, which are unfeasable, which add to the entertainment and which make it more frustrating? Do you expect ES to include any sort of primitive battle system?