If you wanted to make a Civ competitor right now, and I point at Millennia, Ara, Old World, Humankind.
Would you release it missing something like Great Britain and then Sell it later? Or would you probably try to get the best user experience at launch to grab a playerbase?
I think it's interesting that Millennia has few civs (21), despite these being only a flag, name, two city lists (one very short, one a little bit longer), and an irrelevant little bonus. So, for this game, I don't think they were trying to beat the civilization series with civs.
Humankind certainly tried to beat civilization and provided many important cultures that have never made it into a civ game. Partly, because they don't have a good leader that civ could have used (e.g., Harappa, Mycenae, Mississippians). Also, they realized that civ switching required more cultures, so having a large starting roster (60) was kind of necessary. The fabulous continent DLC packs added a whole bunch of interesting civs that never made it into a civ game, and many likely never will. Currently, it has 79 civs in total. I'm sure that one of the ideas of Amplitude was to make the civilization main game rosters, whether at launch (civ 6: 18 or 19) or final (civ 6: 50), look small and satisfy the people which were always craving for other civs, and especially more African, South American, and Polynesian civs – which civ is traditionally very good at overlooking. I think this is also one reason why 7's roster now feels a bit lackluster, even with 39 civs. HK was just very good at delivering the iconic and necessary civs, while also bringing in a whole lot of interesting and previously neglected others. In my opinion, it is clearly the best roster ever in a 4X game (if we exclude PDS games, of course), and 7 has a lot of work to do to come close. HK doesn't have many historical leaders though, but I also think they aren't really missing, because you play against persons or avatars made by your friends (at launch) and meanwhile even yourself (which means you can include any historical leader you want).
Ara also started with an extended roster of 42 leaders, although some civs have two of these. I think they also tried to outdo the civilization series with this. And Microsoft, in my opinion, went for what you describe: the civs make a rather conservative list, the only two "surprises" are Belgium and Palmyra (and that Bolivar leads Venezuela instead of Gran Colombia for some reason). The leaders are for the most part the civ staples and fan favorites. Gandhi is missing, and they added some non-leaders like Sappho (next to Alexander) and Hildegard von Bingen (next to Bismarck). They even added Tamar, the leader that was summoned by fans into civ VI. So, I think Ara tried to grab the player base by having a roster of civs and leaders nearly everybody (or at least nearly every civ player) knows. Whether it worked is unclear – it sold only around 50k copies (on Steam, maybe Microsoft and Xbox are a factor here), but that probably has other reasons than the civ selection. But to me, this looks for sure like the most "play it safe" roster of civs and leaders in all 4X games.
I think one difference is that a civilization main game can in a way afford to have a more interesting roster at launch. Meanwhile, we expect that many civs will appear later on. Some, even some of the most important in history I'd say, have/had a tradition of coming later.
- Isn't this the first time that Maya are in the base game (which I think should always have priority over Aztecs imho)?
- The Ottomans were never in a civ base game except for 5.
- Spain has been in base games (4 and 6), but missing in others (3 and 5)
- Babylon has been DLC in civ 5 and 6.
- The Zulu went from launch staple to DLC
Yet, I think no one was concerned that Aztecs, Babylon, Korean, Dutch or the Ottomans won't make it into the final set of civ 7 – at least before the game released. Now, of course you can label this as a monetization strategy (and it for sure is, and has been for the past 20 years, with a clear tendency to increase), but I believe there is also a big incentive to keep the starting roster at least in parts fresh, interesting, and not completely predictable. And I also believe that the added rule of 10 per era made this much harder to decide. It doesn't mean that the British had to be left out, or that one of Prussia/Russia/British had to be left out, as we concluded here way before release. But it makes this decision easier to comprehend, even if many others don't agree with the outcome. I also believe that the idea of Normans already representing "England" (as we never had Britain before in name, even if e.g., civ IV was certainly meant to be the British Empire and not the English) has some sense to it.