I'm just going to click most of these options.
The very important ones for me are Byzantium, Netherlands, Ottomans, Portugal, Kongo, Mali, Aztecs and Joseon.
Lower down the list are things like Bulgaria (which really needs to make it in on this iteration - THE most important polity in Eastern Europe between the 6th and 10th Centuries), Kievan Rus, The Franks, Flanders (unlisted), Kalmar Union Denmark, Poland, Burma, Muisca, Taino (unlisted), Zimbabwe (unlisted), Swahili/Zanzibar (unlisted), Berbers (unlisted) and Iroquois.
It really is an era that could use many more inclusions. Instead of a fourth Era, they should simply extend the Exploration Era to be twice as long.