Yeah there is tons of stuff in the Battletech Universe that isn't in this game. I think some of it is intentional. Setting the game in 3025 means we don't see anything Clan related or some of the later variant 'Mechs, and I think that's by design. However, there's nothing stopping them from making that stuff available later as DLC or in sequels. The universe has all sorts of weird stuff that this game doesn't have: Quad-legged 'Mechs, a whole set of 'Mechs called the Unseen (lawsuits have held them up), all the Clan stuff that happens much later in the timeline (and is pretty problematic from a lore perspective, honestly), and probably more that I don't even know about.
The Unseen exist in MWO and as models already. The current Harmony Gold caused lawsuit just prevented them being used in this game just in case. Outside the current court case, the Unseen issue is past, with 'Mechs having new art both on the tabletop side and otherwise.
As for Quads, unless MWO adds them (lol no chance), i very much doubt we'll see them here as DLC or even in sequels.
The Clans in particular are something that lots of people remember (due to them being the focal point of the MechWarrior series of games), but if you really remember the lore of those groups it becomes pretty hard to adapt to modern sensibilities. The Clans are essentially all about "pure" bloodlines and natural selection through combat in a way that reads VERY distinctly of white nationalism in today's America. Not that they couldn't retcon them or reinvent them as something else, but at least they have chose to sidestep that issue and all the associated conflicts in this game by not having them in it. Can't say I blame them. You get people mad either way with including them, and by not including them you don't have to address the issues with the old lore.
The Clan blood heritages don't really have anything to do with racial purity, me thinks. Race has never been part of BT really, hell, BT writers have even outlined this really, i believe one book noted "no one would bat an eye at a red-headed dark-skinned rabbi" or some such. The same very much applies to the Clans.
The Clans all about being a
superior warrior, with anything outside being completely irrelevant. Genetics is a way for producing superior warriors consistently, but that is all. Some of the greatest warriors in BattleTech (like Kai Allard-Liao) are natural born humans, a fact even the Clans acknowledge. Yes, they do think Trueborn warriors are better than others, but at the same time a Trueborn failed warrior is less than a freeborn successful warrior, despite the latter not being genetically superior.
For all intents and purposes, despite them being humans, the Clans are aliens. They occupy slot in scifi that is usually occupied by alien aliens. A warlike conquering alien race first and foremost.
(Also never seen anyone compare the Clan "purity" to white nationalism. EDIT And i really can't see how ideas like genetic super warriors are difficult issues for "modern sensibilities". We see that stuff in Halo and other scifi works consistently, and no one ever really complains. It is part of scifi to explore ideas both good and bad, from different viewpoints. And do not mistake exploring a thing to endorsing or promoting it or even just regarding it it a good thing.)
