Personally i would never Combine Eberron as a DM with planescape. It does not add anything to any of the Settings.
Same goes for Athas, being unreachable (almost) adds to ist flavor, making it an easy trip would be like altering Portals in Sigil so that no Portal key is needed anymore!
The way I see it, it's a matter of perspective.
If I'm running a Planescape campaign, I would have no qualms whatsoever about having a portal between Sigil and Sharn or the Eldeen Reaches or whatever, if that's what served the adventure. You might meet some primes there who talk about places like Fernia or Mabar, but those are probably just aspects of the Elemental plane of Fire and the Gray Wastes.
But if I'm running an Eberron campaign, I care about the distinct Eberron cosmology, and I'll likely not have any portals between Sharn and Sigil, and if someone goes to Fernia they are going to Fernia, not to the Elemental Plane of Fire.
The same goes for Dark Sun, of course, though to a lesser degree because Dark Sun is already involved in the larger Multiverse.
Not only that but while the Outer Planes are mostly cutoff from Athas because of the Grey, the Inner Planes are still connected, Athas is still a place that's reachable to any experienced Planewalker.
The Grey was a late invention. It was mentioned a few times in the Prism Pentad novels as a Realm of the Dead (I think originally in The Obsidian Oracle), but nothing was made there of any ability to block planar travel. Adventures and sourcebooks referred to planar travel and to various fiends and stuff showing up. It wasn't until Defilers & Preservers, one of the last books published for the setting, that they said that the Grey blocks planar travel beyond it, both to the Outer and the Inner planes.
Suspecting that Cerilia is unlikely to be reintroduced as a setting, I reintroduce my suggestion of doing Birthright as the next of the WotC D&D board games, having a “Domain Board” with shifting resources and other control elements along the line of both the classic setting and various conquest games. Meanwhile, the game itself would come with an “Adventure Board” with party-level scenarios played out in Cerilia between characters and monsters (which, as in the classic setting, act to affect the Domain Turn).
This reminds me of something Mike Mearls said which might actually indicate that Birthright would be high on their list of worlds to release for 5e: for a setting to be worthwhile, it has to offer something new beyond having a different map. For Eberron, that's the pulp/noir angle. Dark Sun is easy - psionics and survival. Greyhawk... that's tough. I think he said he eventually came up with the idea that Greyhawk could be the gritty world, which would also include things like slower healing and such - but that's putting the cart before the horse, IMO ("We'll only release settings that do something FR doesn't. I want to release Greyhawk, so I'll make something up to distinguish it.")
Under that scheme, Birthright definitely has a strong identity as the Ruler/Domain world. It is the world where the PCs are in charge because of the literally Divine Right of Kings. That is unlike a world like FR where high-level PCs might semi-retire to become lords somewhere - this is a setting where rulership is the main focus.
