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We should try, let's say, a card-based (or mission-based) approach, i.e., come up with a plot for a single map.
I think it all starts with Dionysus, with how he comes to power. It's typical: on the map, there are more than 3-4 computer-controlled opponents (+ about 3-4 neutral castles) - rebels, Dionysus must unite them under one rule. At the same time, he has the task of obtaining a component of the artifact (which one? if it's Erathia, then maybe an angelic alliance?). Victory is achieved by capturing part of the artifact (or delivering it to a castle). You can add a couple of side quests. Aha, and rescuing his friends from the dungeon :)
What do you think?
On this map, it probably wouldn't be a good idea to introduce Apocalypse and his minions. We can do that on the next map and subsequent ones. Each map - one opponent and one hero.