Yeah, I can see that. A more *ahem* primordial setting. Most settings have this hugely long history that trails back for thousands of years. Why not set Mystara as the setting that is just starting out. Everything is new. The Immortals walk the land, setting off all sorts of weirdness. The walls of the world are still very thin and other stuff leaks in all the time. The tieflings could be repurposed as Diaboli quite easily. Dragonborn aren't exactly hard to port into any setting and given the huge number of furries in Mystara, no one will even notice them.
I like this idea.
Mystara does have that ancient history, though. It's just that this is the age when the current civilisation is casting off the fear of the unknown that meant it didn't explore far, and starting to wonder what's actually over the horizon. 'Here there be monsters?' Well, we can kill monsters, and we've proved it. This is the age of Marco Polo, Vasco da Gama, or even Hernando da Cortes, or at least their Mystaran equivalents. Seize your opportunities. Go west, young adventurer! Or South, or North, or East. But, go. YMMV.
And of course, the other place to explore is the Hollow World, where all that ancient history is current history.