There is this HoMM3: The Succession War total conversion mod for HoMM3 in the style of Heroes 2 which happens to have new factions for what looks like Heroes of M&M II.
I've looked into it before but it's just not on the same level of artistic quality and taste as the original.
If you are willing to consider Heroes 3, then Horn of Abyss has fan made factions that are as good as the originals.