Heck yes!
I was always a big fan of the "One Big Multiverse" concept, it seemed to make the whole thing more "real" and fleshed out than a series of disconnected separate worlds. I like the idea that all the common campaign worlds are off in their own crystal spheres, connected by spelljamming and planewalking (and Ravenloft is a demiplane, so that counts as planewalking). The 3e planar cosmology is simply clueless berk nonsense IMHO, the sort of thing they laugh at in Sigil. In my FR game, Oerth and Krynn (and Aebrynis if I really feel like digging up the books) are all out there, and can be reached by astral or ethereal planes (or a proper Plane Shift fork, portal, or other means).
As for languages, I run "common" as being common to a region or continent. FR is my preferred setting, and "Common" is the trade language of the surface of Faerun. There is also Undercommon as a trade language, and every region has it's actual local language (Alzhedho, Chondathan ect.). Elsewhere on the world on other continents, other languages are the common tongue, like Shou or Midani. Planar Common in my game is a pigdin of the elemental and outsider languages (Ignan, Aquan, Terran, Auran, Celestial, Infernal and Demonic), and it's spoken throughout the planes as a trade/common language, and it's also the default language of other intersphere travellers like spelljammers. If someone from Faerun ends up on Ansalon, they'll find their "common" is nonsense, but if they happen to know Draconic, or one of those elemental/planar languages, they're constants.
This "off in it's own cosmology, not tied in any way" bit is one of the things that really turns me away from Eberron, if it has to be shoehorned so much to fit into the normal D&D view of the planes that it isn't the same setting, I really don't feel like devoting time/effort/money to it.
I'd like to tie in Mystara too, but it's cosmology is as wacky and unconnected to the wheel as Eberron, but I'd really like to find a way to shoehorn in Immortals as low-ranked deities and it's cosmology as mostly a network of demiplanes, but that's really stretching things.