Mystara Modern D20/"Shadowrun"
by Marc SaindonOk, so (not really a spoiler), Mystara is established as "Earth along" in the distant past (or at least an alternate magical one) and that it will eventually lose its magic due to shenanigans related to the Nucleus of the Spheres under Glantri City (which shouldn't be a spoiler by now). By fudging geological lore a bit, maybe the ancient site of Glantri City is in modern times under the city of St.Louis.
And perhaps, a group of unlucky spelunkers stumble upon the Beagle, and their messing around with the nucleus unleashes magic back into the world, which creates a chain reaction that re-activates some dormant entities (e.g. Zargon) and signals surviving Immortals, exiled in the Vortex, that Mystara is "back on line".
