Even though AoE II: DE will be coming out soon, I think this time period offers the most in terms of potential content.
-From a military standpoint, it spans the end of antiquity all the way to the rise of gunpowder. From professional armies, to conscripts.
-There are also numerous civilizations (whereas after ~1500 AD most significant nations/empires had been established).
-Combining the two previous points, and you have a huge combination of possible unique units and techs. Even way more than what is in AoE II now
-More geographic reach (i.e. different types of maps). Civs in AoE I were limited mostly to southern Europe, the Fertile Crescent and North Africa because Europe remained settled by mostly so-called “barbaric” tribes
I would still be ok with antiquity, but anything beyond the 1800s is an obvious no-go and would stray from the true essence of AoE