I like the one multiverse approach best because of two reasons:
1) I got my start in 2e, and that was how things were presented then, so a unified multiverse is an essential element of D&D to me, and
2) It allows for there to be a shared D&D setting with a bunch of mini-settings (including home brew ones) that can be experienced either individually (like most standard campaigns) or as parts of the greater whole (like in Planescape and Spelljammer).
While I understand that people who prefer to have their world-specific cosmology refer to the entire D&D reality (such as by having creator deities that created everything that exists, as opposed to this crystal sphere and everything in it) might not find a one multiverse approach as appealing (and I can appreciate that for some that is insurmountable), I think it can work reasonably well by keeping in mind that the Material Plane worlds more or less function as their own...well, worlds, most of the time. These great battles and cataclysms of a world's background can easily play out within the expanse of a crystal sphere--which can be pretty big and easily contain dozens of different planets of various sizes, shapes, and other qualities (as described in Spelljammer), and/or within parts of the Inner and Outer Planes that have a connection to that particular crystal sphere and aren't really anywhere near the parts of the planes that are "mapped" in Planescape so to speak.
The Outer and Inner Planes are infinite. Let's say your world has as its Outer Planes "The Bastion" a lawful good place of order and angels, the "Peaceful Realms" a good plane less concerned with order and more with individuals finding their best afterlife, "The Pit", an infinitely descending spiral of devils, and "The Inferno" a primordial chaotic realm of demons. The Bastion would either be a region of the plane that the classic settings know as Mount Celestia (or Arcadia, whichever fits the theme best) that is an indefinitely large "distance" away from the parts that look like Mount Celestia. The perspective that the plane really
is The Bastion, and maybe off in its infinite expanse there is a mountain plopped down that some foreign gods hang around is just as accurate of a way of viewing it (especially according to Planescape philosophy) as otherwise. In like manner, the Peaceful Realms might share the same plane as Elysium or the Beastlands, while having a completely different appearance; the Pit might share Baator or Gehenna's cosmic real estate, and the Inferno might view the Abyss (if it sees it as relevant at all) as a bunch of distant layers of its self.
The same applies to the parts of the Inner Planes that a world wants to interact with, and the location of its crystal sphere in the Material Plane--they could be so far distant that someone from another world just wouldn't run into them, or they could be right next door--your choice.
Again, I understand that for some it's important that Sigil
not exist in the universe their game is set it (rather than just never come into contact with their world), but for many I don't think it's necessarily the impediment to their goals that it might sometimes seem at first.
Of course, the problem here is that the game has gone back and forth and we all got into it at different times. I can see how it might be jarring if someone started off in an era where the cosmologies were all separate rather than in one multiverse (or there was no established cosmology at all and the idea of playing in a shared settings was abhorrent), and then they stuck them together. I felt similarly frustrated when 3e decided to pull them all apart and kill off the connections essential to Planescape and Spelljammer. Regardless of when we started playing, we got an idea of what D&D is supposed to be established pretty early (not that everyone who started at the same time got the same idea, but there were trends for each phase of the product development) and then they make the ongoing mistake of changing it every edition. I'm more happy than not with the overall way that 5e has done it because it has attempted to be inclusive by restoring the 2e lore, adding in additional lore from 3e and 4e, and including some other innovations. At the same time, I'm still dissatisfied with parts that have been left out (such as the quasi-and para-planes, and good exemplars). I don't envy anyone in charge of D&D today. It is literally impossible to make all of us happy. I think it should still be their goal to try though.
This last part is a bit of a tangent, but another problem with fitting everything in the Great Wheel is magic itself. I read an account from one DM starting an Eberron game that a potential player was angered when he was told he couldn't play a cleric of the goddess Mystra. The player argued that Mystra is the goddess of magic and oversees the Weave, which all D&D magic is based on (BTW, I feel like I recall seeing a sidebar in the 5E PHB that explicitly says the Weave is how magic works), so therefore if magic exists in Eberron than both the Weave and Mystra do, to.
The information has been misunderstood. The sidebar explains that magic is accessed through an interface that "casters have varied ways of naming and visualizing" and that "the spellcasters of the Forgotten Realms call it the Weave and recognize its essence as the goddess Mystra".
It's not that the Weave has been promoted to fill the whole multiverse, it's that the Weave has been classified as the Faerun-specific naming convention for a universal law of how magic works. I would assume it is obvious (but if players are still bringing up the kind of stuff yours did, I guess it isn't) that a god's power does not extend to Material Plane worlds under the sway of different pantheons. Mystra might control this interface (and call it "the Weave") on Faerun, but that has squat to do with anywhere else. I believe what would now be called this interface was supposed to be related to the Positive (Material/Energy) Plane in old Greyhawk lore, and it is the moons of magic in Dragonlance, for example. I'm not familiar enough with Eberron lore to know their theory of magic, but I bet there is something there that fits the bill reasonable well also. On my home-brew planet, it's called the "Breaths of the Dragon" referring to a powerful primal deity.
www.enworld.org
