That is a worrisome thing for fans who enjoyed the non-feudalistic part of the world
Non-feudal societies are still portrayed with very feudal-like structures in CK2. Even nomads have capitals, and tribes are de facto built around "castles" and still de facto work with vassals. Every government type in CK2 is derived from how feudalism is depicted in the game.
So honestly, I really don't think you're missing a lot by not having playable republics and nomads.
On a side note, we also know that there will be at least two tribal types, one of them is clanic (and is active in Scotland).
The nomadic playstyle was appreciated by some players, but it still wasn't great and can effectively be modeled by being a tribe subtype. What we're really losing is republics - and if your complaint is that the game is too feudal-centered, then keep in mind that merchant republics are only an attempt at portraying some of the many largely autonomous Italian cities, that still relied on very feudal-like social structures. If republics relied on derived feudal features in the future, we aren't really losing much.
feudalism is something that was common only in a relatively small part of the much bigger world
That's very debatable, given what parts of the world are on the map. Western Europe had a high population density, and if we add all the different types of feudalism (and related political systems) like Iqta and indian feudalism, we get a very big part of the map - the rest being the ERE, Tibet, nomads and tribes. I know it's quite fashionable to blame eurocentrism and all, but even if it looks small on a map, western europe was still a very important region, and the era covered by CK2 is also the one of the rise and expansion of feudalism. At the end of CK2's timeline, its population is estimated to be between 70 and 100 million people (most of them in western Europe, especially in France and Italy), with the world population being around 450 millions.That's quite a sizable part of humanity, and even if it was about to decrease because of Black Death, it was only the end of a general trend during the middle ages (and we could even extend that trend to the Roman antiquity).
If we also get proper generic mechanics for tribes & nomads, a proper government type for the ERE, and some nice stuff for Tibet, we get most of the map covered.
Let's also keep in mind that CK uses a relatively loose definition of feudalism. We don't actually work with vassal contracts, and we don't really deal with the social consequences of that system. We only play with rulers, very simple vassal mechanics, some buildings and a general army structure. It's not like if we were forced to play with a certain, rigid feudalism type that would be only really valid for a small part of the map.
Which doesn't mean that more government types and diverse gameplay isn't good, but I don't think it's fair to equate a lesser amount of gov types with a less interesting gameplay. You can't just ignore what the devs are saying about republics and nomads and how it didn't interact well with some core game features. You can't say it's poor design. It's a well thought process. CK is about feudalistic societies (including the transition into feudalism from tribal structures), and feudalism isn't just a small part of the map in an ocean of diversity.