HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

ps. if anything were to be taken from World of Xeen, it would be removing one-sided units and restructuring the guild in the first place.

First of all? That would instantly cause an uproar among Fortress players. Because it violates the original gameplay of the game.
And in a way, it's true, because if you do this for one, add transferring money until the first day for another, and it will be OK, the third one needs a bank and so on (otherwise it's NONSENSE!). And the fourth one doesn't need any of this because "it's good enough," Fortress classic forever, if you want these features, then play World of Xeen. So, it's a reason for a flame war.

And the main breakthrough would be simultaneous turn-based gameplay, not the papal mirror effect.

Simultaneous turn-based gameplay would save 2-3 real-time hours in a game, and that's a lot. And it would be very cool. But there's something "not quite right" with the exchange of technologies :D