I do not want to make something like Age of Empires, I plan a different system. I played a lot AoE2 and AoMythology, but fighting against skirmishing units one by one was annoying for me. in other games it was solved in a better way, by using only groups of fighting units. basically it is a very difficult genre, it would be very complicated to make a smart AI adopting to the ever changing environment and plan unit creation and movements effectively in real time, especially on one huge map. I build my map system to be able to manage real time changes, but I know alone it would be a too hard task to implement a great game play, so I will somehow combine it with turn based elements, by separating building, development and diplomacy from battles, like in Total War games, and by using more map levels.
the selection of the era will depend heavily on my interest I think and on how I can represent the actual military technology. the easiest is infantry, I mainly deal with it, and have a lot of optimization ideas, but presenting nice tight formations behaving intelligently is not too easy. cavalry is a bit more difficult, but I also have a prototype system of it, by simulating movements using acceleration/deceleration and limiting turn radius by the pathfinder. slow siege equipment are also not a big problem. but fast vehicles (like chariots and tanks) are what I want to avoid if possible. imo they would be very performance costy thus would decrease max. unit quantity a lot, and would require an effective unit type priority system to move away low priority units from they ideal path that is sometimes covered by other own units. so what are the most undesirable from the point of view of development are large fast units and huge crowd armies. and I will need later to deal with ships too (what is simply omitted from some games).