Plot

Catherine's sister is having problems; perhaps her niece? We need some female relative to lead the former Erathia. Nicholas isn't founding anything new; he's staying in Axoth.
The world isn't Ksyn, it's Enroth – good, I'll rename it, and there's no need to repeat it; I understood it the first time.
The outlines have indeed changed – there's no longer Antagarich and Enroth; there are three continents – Antagarich, Enroth, and Jadame served as the basis, but only as a basis.
The same goes for the Vor (their island has decreased in size and is now called Scandia; some other races have also joined them, such as the dwarves from Avl, etc.).
The Mongols in Enroth also possess magic; in the magic-infused lands, their shamans have gained great power and can summon Tengri – messengers of the sky. They found the Nomads race in the south of the steppes – unsurpassed riders, plus their numbers, plus discipline, plus their ability to wage war; we shouldn't underestimate them.

It's worth adding a point about the fact that after the Reckoning, something came to Enroth from that fiery world (HoMM4).

– Do you mean the world where Gaulot traveled, searching for Kalibar?

Let it be an entrance to "Atlantis – a world of water," rather than an entrance to "Atlantis – one of the realms of the Sphere of Water."

– Okay.