My personal heresy:
Reduce the number of Upper planes. There are currently 17 planes for nine alignments. Furthermore, the naming conventions could use a reexamination. Several planes still come from existing/historical faiths. DnD cosmology should be just that, DnD cosmology. Pastiche is fine enough, but outright embodiment of real world beliefs is challenging, if not for the simple reason that even with a bloated 17 planes, we don't even come close to representing all world religions. They changes Nirvana into Mechanicus. They can do more.
They're not going to reduce the number of upper planes unless they get rid of all the dual-alignment planes, because they like their symmetry. Which, theoretically, they could do--they've basically ignored all the lower planes in favor of the Abyss and Nine Hells already. Since they have already blended the para/quasi-elemental planes into the basic four elemental planes, I wouldn't be surprised if Pandamonium, Carceri, Gehenna, and Acheron got blended into other planes: Pandemonium (and Ysgard) and Carceri (and Bytopia) can become part of Limbo and Mechanus, respectively, and Genenna and Acheron can become part of the Abyss and Nine Hells.
I'm of two minds about that, myself. I actually prefer those dual-aligned planes to the Abyss and Hells; I find them a lot more interesting and wouldn't want them to be removed or mixed in with another plane. Heck, I'd be happy if the Abyss and Hells were totally downplayed, simply because I find them really boring (I know they won't be, though). On the other hand, I know that, if I were to run a Planescape game, I'd completely change the planes around to be more to my liking
anyway, and my idea is closer to what I think 4e did, with individual domains claimed by deities and arch-whatevers or just naturally-formed bits of weirdness, than anything like the Great Wheel. After all, I don't need both a Limbo
and an Elemental Chaos, after all. But that's me; probably some people like them both.
While there are still planes and layers named after parts of real-world religions and mythologies, I think it might be more important to simply remove the real-world
gods from the setting. Or at least the real-world gods that haven't been co-opted into a D&D religion, like Bahamut and Loviatar. What I would like to see is them come up with a brand new bunch of gods to fill in the blanks where the real gods had been used previously. Not just renaming the real gods into something more fantastic, but literally new gods.
(Sadly, though, the book is going to be 96 pages long; there's no way they'll come up with a new grouping of gods, even if they only gave them each a 1-sentence description.)