remake

In the Heroes 3 editor, you can create a non-linear campaign, but I can't imagine how to implement the transition from dark to light factions.

This is quite easy to do; you just need to experiment with the map settings and the settings for transitioning heroes. For example, in the last campaign of Shades of Darkness, there was a campaign where you could play as Rangers, Barbarians, and Necromancers. At the same time, no one lost experience when transitioning.