Which Might and Magic universe are you referring to?

I believe that the only way to connect two worlds is to have them exist in the same galaxy.
I'm not sure what worlds were in the old universe, but there were portals to Axeoth, so maybe there could be portals to Ashan (as someone here wrote) - thus, heroes from the old universes could have ended up in Ashan. But this does not negate the uniqueness of Ashan's culture and origin.
Why tie them to some Ancients or whoever. Let them exist side by side =)

Bobchik, the problem is that you don't know. Read the history of the old Universe. All the worlds (Enroth, Axeoth, Xeen, Terra, etc.) exist in one common space (in one galaxy or in different ones - it doesn't matter). Thus, interstellar travel between planets = a portal between worlds. And the Ancients are the very race that populated/transformed most of these worlds. They left the portals and spaceships. And also, in each world, the Ancients placed their Guardians, who in Ashan can be Ashan and Urgash. This is about the connection between the worlds.

If we talk about the fact that in Ashan we have a division by races, and not by castles, then let's go through it in order.

1. Castle-Haven-Haven. There is nothing to say about the comparison between the third and fifth versions, the swordsmen were replaced by griffins, and that's it. In the sixth version, the pikeman was crossed with the crusader, freeing up space for a healer, although she is also a human. But the mutation of the monk into a Gloria is inexplicable, as is the transformation of ordinary horses into solar steeds. Why - it's not clear, but never mind. In the sixth version, the number of races in the city increased by one - Gloria, so we get 4 races in one city (humans, griffins, angels, Glorias).
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2. Rampart - Sylvan. The main thing that people like to talk about is that instead of dwarves, there are elf-swordsmen. That's right, in the third version, the dwarves lived underground, as they should, and, having concluded an alliance with the elves, they arranged an exit from their underground mines in the Rampart, everything is normal. In the fifth version, the elves and dwarves are not allies, and the Sylvan castle is a huge tree, there is no place for dwarves there. In the rest, the centaurs became fairies, the riders on pegasuses drank away their pegasuses and went into druidism, the rest remained unchanged. That is, from this point we get -1 race.
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3. Tower - Academy. The Nagas became Rakshasas, the rest remained unchanged. So, in terms of races, the score is 0. There was also a claim that the mages should live in the desert, and not in the snowy mountains (yes, Hel (or is it still Xel?), I know that there are deserts in Erathia, but this is not visible in the third version, and we only see this in Might and Magic 7, so let's skip this point). But, excuse me, why? Explain to me why mages should live in the desert, and not in the mountains?
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4. Inferno - Inferno. The transition from the third to the fifth version was again marked by minimal changes. The Gogs were made into succubi and swapped places with the demons, instead of ifrits, nightmares were hired and swapped places with the pit lords. Plus or minus, it's all the same. In the sixth version, let's look. The trio of Imp-Gog-Cerberus turned into Frenzied-Succubi-Cerberuses, that is, again, it's all the same. Further, yes, "changes". Demons, Pit Lords, Ifrits and Devils turned into Broodmother, Scourge, Ravager and Pit Lord. In general, the old Demon and the old Pit Lord (Spawn of Evil) are similar to the Scourge and the Ravager, the Devil is the new Pit Lord, also plus or minus the same. That is, we have a replacement of ifrits (by the way, according to the history of the third version, they are mercenaries in the service of Krigan, and not Krigan himself) with the Broodmother. Again, it's all the same. Ah yes, the unique "style" compared to the third version, which NiKShoKer mentioned. In the third version, everyone was, as it were, "humanoid with horns". Of course, red humanoids (exception - dogs and broodmothers), all burning after upgrading and all having horns or spikes - the originality of the units, uh-huh. The height of originality.
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5. Necropolis - Necropolis. First, the Death Knights were replaced by Undead, there is not much meaning in this, well, whatever. Then they were replaced by some Lamassu, and the Bone Dragons by Fate Weavers, and here a question arises - why? In a castle of the dead, dragon-undead fit better than mutant bitches. Lamassu is generally an incomprehensible thing, if Death Knights are clearly undead from Cavaliers, then what is Lamassu? Where does such crap live? They are made for show, they say, "we are not copying everything, we can do this too!"
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6. Dungeon - Dungeon. As already mentioned, a set of magically bred warlocks' creatures has been replaced by a relatively homogeneous army of dark elves. Is this good? The answer is ambiguous. On the one hand, fans of Dark Elves are delighted, and the army has become less diverse, on the other hand, it somehow looks illogical, to be honest. If the Dark Elves are so proud and cool, then they, like the warlocks, should not fight themselves, let the slaves go ahead, and this is also said in their history. But in theory, in the game, the Dark Elves of the first two levels act as meat, and only then do the minotaur slaves appear. But this is my opinion. Also, I agree with Xel and believe that the Dark Elves of Jadama are much more original and interesting than the D&D and WH calque.
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7. Citadel - Stronghold. There was a claim about exclusively orcs. You know, NiKShoKer and bobchik, in many universes there is a term "greenskin", which unites goblins, orcs, ogres, because they differ only in size and physique, character, skin color, culture, everything else is common. In the third version, we see exactly this union. Two goblin units, orcs, ogres, and with them Rukh birds, cyclops, monsters. In the fifth version, we have a goblin, a centaur, three orc units, a wyvern, a cyclops. That is, there are again 4 greenskins, plus appendages. And if you take into account the size of the executioners, then, to be honest, when I didn't see him in the game, but only saw the first screenshots, I thought that this was an ogre... In the sixth version, the situation is the same, only a harpy instead of a wyvern.
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8. The swamp castle from the third version, Conflux, the dwarf castle from the 5th version, the Naga Sanctuary. These are unique castles and it is difficult to compare them properly, not with each other.

In general, I wanted to say. Regardless of the part, the castles are more or less the same. And this "more or less" is very small. It makes no sense to say that one is good and the other is not, because it's all the same. The exception is the radically rewritten underground castle. And it will always be so. Otherwise, in order for the castles to start to look different from themselves from previous parts, you will have to do a big experiment, and the fourth part is an excellent example of this. But the publishers will not go for it - the risk is too great.