Didn't say you should but it'd be nice if even one example could be pointed to as an example.
Excellent, then you'll be pleased to know others have stepped up to the plate.
I would tentatively nominate GURPS here, along with Amazing Engine.
As for...
But these don't change the fundamental way the game is played...
How is the playstyle of using Tome of battle different from the playstyle of regular 3rd edition? It offers a different type of martial class but that's no different than having the spellcasting differences of a warlock vs. a wizard.
Epic Level Handbook... Not too familiar with this so I'll ask... how does the fundamental playstyle change when using this book.
Psionics/Incarnum/etc. Again add player options but don't change the playstyle of the game.
Same with the campaign settings mentioned... the playstyle isn't really changed... Maybe we are talking about two different things here.
So...
Part of your initial enquiry was:
EDIT: To better clarify what I'm asking... is there an example of a ttrpg that is modularly designed to accommodate *through robust support) a multitude of playstyles?
To my mind, there is nothing in there that suggests the game had to
fundamentally change its nature. So you'll have to excuse me for feeling a little grumpy that this additional criterion is being brought up.
Also, I'm not sure that even would be desirable: when you play GURPS or D&D, there is probably still an expectation that you are sitting down to play
GURPS or
D&D, even if you are sitting down to play D&D. In. Spaaaaace! (aka Spelljammer) versus when you are sitting down to play Grimdark. D&D (aka Dark Sun). There's still a reasonable expectation that both of those games should recognisably be D&D to some extent, even if there are also differences.
At any rate, I would posit that the gameplay introduced by ToB or ELH, even if they aren't
fundamental (and in some cases they may well be), are still different enough from the core gameplay of the 3.5 PHB that they could (and did) satisfy the gameplay preferences of player constituencies that WotC was interested in retaining and whose preferences were "under-satisfied" as it were, by the core rules.
For instance, in the Epic-Level Handbook, the rules for
epic uses of the Balance skill provide for balancing on surfaces less than 1 inch thick (including hair-thin surfaces) or even for balancing on a cloud. These use cases of Balance are still using the 3.X mechanics for skills, so in a very real sense the
fundamental gameplay hasn't changed. But even so, the epic-level "module" has something to offer for those players who want an "epic" or "mythic" gameplay experience outside of what is offered by the core rules, while still recognisably being "D&D" (in its 3.X form).
As an example of what have in mind for how D&D could be designed with more of this modularity in mind, I would assert that, for instance, D&D could probably dispense with most of its mundane equipment being
explicitly listed as a "core" rule element in the PHB. For what I suspect is the largest player constituency of D&D these days,
it just doesn't matter whether you have "tent" written down in your inventory or not (and so on and so forth with most, if not all, regular items), and
it just doesn't matter how many pounds of stuff you can carry - up to a point: most players, I expect, will probably happily concede their barbarian can't carry a 10,000-pound golden statue all by themselves.
Now, it may seem odd for me to be in agreement with
@Micah Sweet but, like Micah, I would personally find this unsatisfying. I like a bit more heft to mundane equipment (albeit maybe not to the same extent as Micah). Instead of having the current set-up, where the PHB is littered with vestigial gear, I'd rather have a core rule as above, a variant rule in the PHB that adds some of the heft I want (like an abstracted slot encumbrance and "Gear" item that takes up space in your inventory, that you expend as you use it and as time goes by, but lets you abstractly handle some details of inventory management) and a second variant rule that has a more fulsome list of items probably not far off from what is in the PHB now, which may or may not include some descriptions of how they're typically used and/or (à la Torchbearer) at least one concrete benefit for bringing each such item along on adventures. That way:
(1) The largest player constituency or constituencies get what they want -
not to have to bother with inventory management at all;
(2) I get what I want, or at least something close to it;
(3) Micah Sweet gets what they want, or at least something close to it;
without either (a) the rules being a wishy-washy compromise that don't particularly satisfy any of us or (b) us having to squabble over whose vision for inventory management ought to prevail.
I'd be willing to bet that the space savings from cutting down on "core" equipment for group (1) frees up almost enough page space to add the content for group (2) and (3) without having to add too many extra pages.
Meanwhile, the DMG could include some discussion about why you might choose the "standard" rule over one of the variants (or vice-versa).
Now, would this be
easy, especially once you start taking other aspects of the game into account? Certainly not! I am sure that in many cases bland or unsatisfying compromises would still have to be made. Would it be
possible? Well, I don't think we'll really know until WotC tries. Would it be
successful? I fear that for that to happen, a lot of us would have to be willing to put aside sensibilities of exclusivity in ownership.