This is something we've been exploring in the Parallel Wheel thread, but it's come up in various places.
Just as you can have the Wheel without Planescape, do people think you can have Planescape without the Wheel? Without alignment? With different ideas about Good/Evil and Law/Chaos?
Beyond Countless Doorways offered some possibilities, but I'm curious about what people feel defines Planescape, if they've played what they felt were PS games in different cosmologies.
Personally, I feel like playing a Mage: Ascension game can have a lot of PS flavor, even if you work on Earth. Funny thing is I don't know how to define PS exactly.
I also think you could have alignments but with more planes you have more options. So planes of Law are "closer" or even coterminous with each other (if not the same place from different viewpoints), and this would get around the challenge of making a place like Acheron or Beastlands the sole incarnation of their particular alignments.
I think a plane like Mechanus would benefit a great deal from this, as would Limbo. Other planes like the Abyss are incredible inclusive, with layers representing all kinds of possibilities for CE. I feel like you could have every plane like that, blurring the distinction between layers and planes.
Two things:
1) There's a systemic organization to the cosmology. Obviously alignments are the big ones, but I don't think that it has to be that for me. Elementals and genies certainly understand good and evil and law and chaos, but those are not the organizing principles of their world (and I suspect that to an elemental creature, the tension between fire and water is about far more than the obvious physics of it all).
2) Building from there, what might be the most special to me about PS is that it takes the coexistence of all these different planes and beings and whatnot very seriously. Which is to say that while the intersections and points of contact can be fascinating, there's something mundane about them as well. Planars are just trying to get through the day, you know? Rip's writeups of Abattoir and Void's Edge totally nail this for me - they're exotic and scary and full of imagination, and this isn't in spite of them being relatively straightforward reportage of the day-to-day in these two cities, but precisely because of it.
Maybe I've wandered off track here a bit. PS is great. That is all.