If they were going to introduce entirely new concepts then they might as well have not bought the franchise in the first place but just made RADIOACTIVE.
Here's the thing: a unique setting in an established universe necessarily means, to my mind anyway, that you should be exploring a new area, or pointing out
what makes this unique, or otherwise make the content that's reused, reused in a way that implies progression of the setting.
Fallout 1, 2, the unreleased Van Buren, and New Vegas understand this. While each takes place in a unique setting, they still reuse the core things from the base game, AND make sure that the historical progression of the setting feels realistic.
Fallout 1 as a setting is entirely unique, since it's designed to be an introduction to the universe. The Brotherhood of Steel is a unique group that shows up around Lost Hills, the Ghouls are the unique mutants from Necropolis, the entire creation, purpose and ideology of the Supermutants is a core part of the game's story.
By the time you get to Fallout 2, it takes place in Northern California, which is a new setting to the SoCal based Fallout 1, while still having heavily overlap in the southern parts of the map. This is weaved in to the way the game is designed: It takes until the 6th town your average player will visit to see Ghouls for the first time, and other features from Fallout 1 are used sparingly:
The Brotherhood of Steel have a minor presence mostly comprised of outposts in major cities, where they're subtly nudging you on your way. There is one main reocurring Brotherhood character who shows up in a total of 3 locations. This is a very small-scale northern expedition, by a group driven to rumours of Enclave presence, this isn't a whole new chapter or invasion force just for this area. Vault City is the game's new Vault, but even then it's entirely written to be a unique town/faction in the game's world.
By the time you've seen your first Ghouls, you've effectively got 90% completely original content, with a whole new species of underground dwelling mutants terrorising a town, getting involved in gang-wars, seeing Northern California's slave-trading economy up close, ect., most references to the original game are portrayed through this lens of being far away from where it originally took place: The Vault Dweller is mostly a mythological figure this far north, people remember seeing vast hordes of Green-skinned Giants migrating across the wasteland decades ago, but have no idea what any of it means.
Moreover, when you do see things from the previous game, their presence is a direct result from their presence in the previous games: The Ghouls in Fallout 2 are a result from the "Great Migration" from Necropolis, where after the Mutants destroyed the city in the first game, they've been forced to flee and settle down across the wastes. FEV, a core part of the Enclave's plans, is directly derived from Mariposa, which they had to excevate using Slave Labour due to it's destruction by the Vault Dweller, and this excavation creates Second Gen Supermutants. First Gen Supermutants are now living in towns and cities, trying to get by with humans and use their unique abilities for the better, since after the Master's death, they are no longer the big bads, but rather the remnants of an army that no longer exists.
Van Buren was going to have a similar attitude: Almost all of the game's content would be unique never before seen locations, ideas and groups of survivors. The parts that were repeats, were again, part of a progression from the events of the first 2: The Brotherhood were almost wiped out in a war against the NCR over the technology from the Enclave, and a small faction of extreme loyalists, the Circle of Steel, has gone East to make preperations for a recapturing, Ghouls play an incredibly minor role, and all of the game's Ghouls come from the Reservation, a facility known for producing "Born Ghouls", who were born as the result of Ghoul childbirth. In dealing with the Reservation, which effectively serves as a Theocratic Dictatorship with a slave-economy, you'd have to take in to account the fact that in previous games Ghouls were infertile, and would likely be the last generation of their kind to ever live. The inclusion of Ghouls in the game
at all was going to be justified through a moral choice about the very existence of Ghouls in the Wasteland.
Every core theme that makes Fallout what it is, is important
BECAUSE it's intentional and well thought-out. When the game includes features from the old games like Ghouls, Supermutants, the Brotherhood of Steel, you have to ask
why they're there. And not just "The lore said this"
Does it feel like a natural progression, whereby the events of the previous games directly made the faction who they are today?, Does it feel like they're raising new and interesting questions about the nature of these things?
You can have as many new sources of Supermutants as you want for every single game that comes out, but what makes a Supermutant a Supermutant is the sense of historicity. Supermutants exist in a historical context, and each new iteration of them should feel like a progression of the last: How are the Supermutants doing since the events of the previous games?, What has changed since the world has changed around them? If there's no history to Supermutants, why bother making them Supermutants at all? All they become are another enemy, that's sole similarity with Supermutants is the name and size.
Fallout shouldn't just be a checklist of tropes like "Ghouls, Supermutants, Raiders, Brotherhood". Each of these things should have some purpose, and some real feeling reason as to why they're there.
