I actually don't think the "real world cultures in a fantasy setting all together" is an actual hook.
Take a look at the similarly titled Realms thread and there are people mentioning the "there's renaissance europe, medieval europe, celts etc all in one place".
IIRC, the Realms were created as a "fantasy Earth" anyway. In fact, this theme can be seen in any number of worlds out there. This isn't unique to Mystara by any means.
As an outsider looking in, it seems to me that Mystara could actually use a bit of a reboot. The setting comes across as kind of a hodgepodge of ideas. I think I would play up on a few things.
1. Hollow World. This is just a cool concept.
2. Red Steel/Cinnabar (sp?) - This comes across as Mystara's own cool power system, ala the spellplague or dragonmarks.
3. Skyships. The entire reason I bought the Champions of Mystara boxed set. It went right along with my Spelljammer stuff. In fact, I'd probably borrow heavily from Spelljammer to flesh this out.
4. "New" races. Rakasta, Lupins, Tortles, etc. New races help to flesh out a setting.
5. Futuristic technology of long ago. This can be a cool draw, and no other D&D setting seems to use it. I'd consider tying in Tale of the Comet.
6. Immortals. This just has coolness written all over it.
7. An event. I think Mystara could use some major event to move the timeline forward, and to help provide a unified setting.
Again, this is an outsider's point of view. I don't know the setting like some folks. I think that this sort of focus could help provide Mystara with that much-needed identity. It wouldn't necessarily be the Mystara of old, but maybe the time has come to really redefine the setting.
Or should the Realms "kill Mystara and take its stuff?"
*shrugs*