Of course the trouble is, the way WOTC did the settings probably would anger a lot of fans.
For instance, look at how many people are disappointed with the job D20 Future did with Star Frontiers. Not quite the 'mob with torches rattling the gates of the castle' level of mad, but disappointed.
Or the way that Dungeon/Polyhedron butchered Spelljammer. Yeah, many non SJ fans liked it, because it took out most the weird stuff. But SJ fans liked the weird stuff - that's why they were SJ fans.
Anyway, also, while technically Mystara was both the Known World (the outside of the planet) and Hollow World (the inside), I think both are large enough that you couldn't really do them both in one book. The style was also somewhat different - the Known World was a fairly standard, if extremely high level and high magic setting, while the Hollow World was largely based on real world cultures. While it wasn't exactly low magic (the Eygptian like culture had magical hover tanks, sorta), it wasn't quite as high.