But in general, it's nice to see this much variety in the 4x space. Within the space of a couple years, we'll have had Old World, Humankind, Millennia, Ara: History Untold, and smaller ones like Hexarchy and Ozymandias.
While there is a variety of Titles, unfortunately (barring ARA from this, because I simply haven't had a lot of contact with it) there is not a lot of variety or originality in how they handle the mechanics of the 4X experience.
Combat. Old World and Civ have 1UPT. Old World makes it more dynamic by having variable movement rates and very long movement rates compared to Civ. Both of those are old boardgame mechanics: variable movement was in Avalon Hill's
Afrika Korps back in the 1960s, long movement was a feature of
Panzerblitz from 1967. Millennia and Humankind use a 'drop down' battle map/screen to resolve multi-unit battles, but Humankind actually lets you resolve the battles while Milennia only lets you watch: Millennia's system is ugly to look at and gives the gamer no agency. Humankind's stops the entire turn while you resolve a mini-game battle - and in the late game may have to do that several times each turn, slowing the game to a glacial pace: I personally once spent over an hour resolving a bunch of late-game Humankind battles all in a single game-turn, and it was one of the last times I ever opened the game: life's too short.
Resources. Everybody gathers resources in some way from the map, everybody has those resources fixed on the map despite the four-legged fact that sheep, cattle and horses are migratory animals and the essence of agriculture is to plant crops in new places to make it easier to feed the multitude. Millennia has Production Chains to use the resources with some more depth, but Production Chains have been more extensively done in City Builder games going back to the old Sierra titles and in all the Anno series.
Tech Trees. Everybody has one, even games on a smaller timescale like Old World where many of the Techs provided were in reality already well-known long before the timeframe of the game. Millennia's set are much more abstract than Humankind or Civ, and slightly less linear than the other two, but the Techs still largely come in a vacuum, undisturbed or affected by any thing else, like terrain, climate, or society. Civ VI introduced Eurekas to touch on that, but the connections between Eurekas and the Techs are all too often emphemeral. There is still nothing, in any game, that comes close to recreating the wildly different technologies developed in Europe/Mediterranean and China from 400 BCE to 1300 CE, and in all the games Tech becomes a race to stay ahead or even: technological backwardness is fatal in almost every game.
Culture/Politics/Social Systems. Millennia's is the most open-ended. You are tasked with building your Nation into Whatever You Want, so are given almost no starting bias and potentially have access to any combination of traits. Unfortunately, that makes all the Nations indistinguishable and even in the limited 60 turn Demo, bland. Old World, with its tighter focus on a limited timeframe and geographical set of factions, has a resulting much more detailed political/cultural game, but also has only one type of social/political structure: the dynastic family, and so has to take groups like the Classical Greeks and Romans who got rid of kings early and force them back into the dynastic mould to put them in the game. Civ gives you starting traits and the infamous fully animated, voice-acted Leaders, which both limits the number of Civs in the game (because 3D and voice acting are Resource Sinks) and defines to a considerable extent what kind of Civ you will play, a severe limit when trying to depict civilizations whose character has changed dramatically over the centuries, like China, Egypt, or Britain. In Civ VI, you do not play a Civilization, you play a slice of a Civilization. In Humankind, you also play a slice of a Civilization, but it's usually a completely different civilization every Era/Age, which becomes difficult to keep track of and massively annoying after a very few games.
In short, nobody has really got the Grand Strategy 4x historicalish game right yet, despite over 20 years of trying. Old World comes closest to achieving the 4x goal, by limiting the scope of its attempt to a single Era and a single geographical region, but that's cold comfort to those who want to play the Great Game spanning thousands of years and the entire planet.