One of the spins I put was the wreckage of a fallen multi-dimensional sky city in the northern parts of the Principalities of Glantri.
The PCs never visited it, nor did they follow up on it. I'd largely forgotten about it until I started re-reading Michael Moorcock's "The Knight of the Swords."
Here was the weed-grown wreckage of the vast sky city that, during the month-long battle [. . .] had careered from one plane to another, rupturing the fine fabric that divided the different dimensions of the Earth until, crashing at last upon the gathered ranks of the Vadhagh and the Nhadragh, it had destroyed them. Bring of a different plane, the tangled metal and stone of the sky city still retained that peculiar shifting effect. Now it had the appearance of a mirage [. . . ]- Michael Moorcock, The Knight of the Swords, First Book of Corum
The book states shortly after that the ruins look differently viewed from different dimensions, and they are sprawled out in an area 20 miles in circumference.
That description really grabbed me. It also reminded me then and now of one of the ghostly "wrecks" in Stewart Cowley's Spacewreck. So I had to put it in.
I'm not sure I ever had written notes on what I intended to do with it - I may have had little besides a description. But I sure would have done something with it eventually.
But it started life like a lot of my ideas do - vague sketches based on bits I liked elswhere, as stub-ends for eventual adventuring use.