Meh.
I'm not really personally interested in mashing together the settings for some cornball tourism. Almost every time it is done, it is done poorly ("WHAT IF DRIZZIT HAD TO RIDE A KANK THROUGH THE STREETS OF SHARN YOU GAIZ?!", or when FR invaded Eberron in DDO), and though mashups can be done pretty well, the mashup itself becomes the setting when that happens (see: Kingdom Hearts, arguably Planescape/Spelljammer in some modes). A game hopping from experience to experience like that would get so schizophrenic, it wouldn't have time to be enjoyable. No chance to feel out, say, the deep history of FR and how it affects the present, nope, just do some ruins, talk to some elves, and its on to the next thing!
For me, each of these settings offers a unique way to play D&D, a different direction to take your game experience in. You could spend years in the sands of Athas or the City of Doors and not explore 1/10th of what those settings offer. Why cheapen it by putting it in an It's A Small World ride that the players can zoom through? These settings should be deeper experiences than that, experiences that allow you to see the variety and complexity of the creatures, villains, heroes, and locations therein.
It might sound nice on paper, and you could pull it off, but I don't think I'd be interested in it, and there's a HUGE propensity for screwing stuff up while you're at it. And for what reward? A montage of IP names?
Pfffft.