Since AoE 4 is a return to the Middle Ages, then probably AoE 5 would be a return to early modern (since ancient will be added to AoE 2). However, I would prefer a completely new antiquity game to be created.
Games about modern conflicts (from the 2nd half of the 19th century) due to the fact that they should contain completely new mechanics, should be a completely new Age of Empires sub-series as well as Age of Mythology.
I already proposed with @GusTank04 a completely new series of games - Age of Wars: