Wait, what? The freedom to mold reality by force of belief is a limiting thing? That's pretty much the 'anything can potentially occur' stamp on a cosmology, and that's a limit to what you could do? I'm confused. It's a lot more open to change and diversity than a much more limited, static system.
I'm not TwinBahamut, but I'm curious as to how "reality not defined by belief" is by necessity more limited and static. The next step, after all, is asking what reality is defined by if not belief. Moorcockian powers of clashing law and chaos? A Celestial Bureaucracy with a thousand separate sub-ministries feuding for prestige in the eyes of the Jade Empress and the according power to extend their influence? Raw science? Whatever your answer, that will define whether the system is limited and static.
From my time in the old World of Darkness, I can report that a reality shaped by belief tends to run into troubles with other cosmological tropes you might want to establish. When I was helming Werewolf: The Apocalypse, I frequently found that I had to outright ignore or disagree with Mage: The Ascension if I wanted the themes of Werewolf to come through. Specifically, Mage had a very strong "humans are the most important creatures in the universe because their belief shapes the universe itself." Werewolf, on the other hand, was about "Humans are not the most important things in the universe, and in acting like they are, they're doing a lot of damage to everything else." You probably wouldn't get the same problem in D&D exactly, what with the consensus winding up including humans, gnolls, neo-otyughs, aboleth, couatl, devils, etc. (It would raise a lot more interesting questions, though. What if human belief is a minority? What would a world be like where what humans believe is not strong enough to overcome what the more numerous orcs and goblins believe, or what dragons believe?)
Having said that, I've got no trouble with consensual cosmologies myself. I tend to personally lean, however, toward cosmologies with certain bedrock truths that then have a lot of different manifestations and facets based on people's belief. An Underworld that is always the Underworld, but might look different depending on the local cultural touchstones. Gods that are pure archetypes yet wear different masks for different cultures. Things like that. I like to monkey around with the idea of a cosmology that mirrors different beliefs without having those beliefs actually define it. It adds an occult layer to things, differentiating between practical truths (crocodiles are sacred to the god Hedretha, and some may speak with his voice) and more fundamental truths (Hedretha is but one face of a powerful god of implacable nature; the Horned King is another).
I liked Planescape quite a bit (though I think it had a few flaws, such as the very specialized belief systems of the factions). I do have to think it was very baroque by design, though. If people sat down to create a D&D setting about consensual cosmology, but they didn't have the design tenet of "make it work with the pre-existing Great Wheel setup", I don't think it would look much like Planescape. Planescape's not really intuitive. It's quirky. That is, of course, one of its great selling points. But I quite understand why designers might want an alternate, more intuitive cosmology. Even if some of the names are a bit silly.