My idea started out both simple and challenging: "What if I ran a D&D game with as much surreal, plane-travelling weirdness as there was in late-90s Magic: the Gathering. I'm talking the peak insanity of the era of Tempest, Ice Age, Urza's Destiny, and Urza's Legacy, where the game was simulating a war between insane cthulhoid cyborgs and a race of precision-bred magicians with living shards of crystal and an inter-dimensional goblin horde in the middle.
I wanted low-level PCs to be roaming the planes, dealing with truly out-there wizards, and warped magical entities. As always, I subscribe to the theory that level one should feel as awesome as level 10.
My Setup
Swords & Wizardry played RAW, but using a cosmology based on the AD&D2e / D&D3.X era Great Wheel / Planescape planar layout.
The PCs come from a kingdom on a mostly-undetailed world I am basing mostly on a mix of H.P. Lovecraft's Dream-Cycle mixed with a bit of Lord Dunsany, then brought kinda' down-to-earth with the trippier bits of The Elder Scrolls' Tamriel.
At the beginning of the campaign the PCs are new arrivals at Armathan, a grand castle on a high-mountian meadow. The Mountain's great workshops are powered by a waterfall and streams that run right through its undercroft. The Castle itself was built as a part of and to defend a silver mine and an important monastery. The mine in turn connected to ancient pre-human ruins.