My guess is that such a slipcase wouldn't cover Sigil AND the Great Wheel, at least not "properly." I think it would look something like this:
Sigil book - details Sigil, the Outlands, and Factions
Adventure book - a series of maybe a dozen adventures, about half of which are focused on Sigil and the Outlands, and the other half as forays into various planes - so there would be info on some planes, but not the whole shebang.
Splat - Guide to character creation, options, maybe some rules on planar travel.
Full coverage of the planes would have to wait for a hypothetical Manual of the Multiverse book. Or some such.