In the
Monster Manual, cloud giants are "Usually neutral good or neutral evil," so take advantage of that: they can be both helpers or enemies. They should be both, at times. A faction of helpful giants...and plenty of bad ones.
Stormcloud giants.
And you can bet that the bad ones probably see Aeshotal as an iconic hero of giantkind.
Yeah, I'm not wild about elves just getting bored with the forest and taking the sky (though I'm sure there's a little more to it than that). Tragedy always begets cool things, good and bad. So go with your idea that it
began with the War of Divine Right. Are sky elves winged, or do they just have magic of flight?
Here's another possibility. Most elves revered the goddess Ireth, the Starmistress, deity of forests and the night sky. What if, during the crisis at the end of the War of Divine Right, elven priests of the god Shul or perhaps even Madrah (they'd have a different name for him, though), became suddenly outspoken. They used all the power at their disposal to do just what you were suggesting, take pieces of their dying cities and float them up—lest they be swallowed by the fire and the sea.
So you could have little understood magic, the same kind that floats the cloud giants' castles, hold aloft remnants of Ahna-Vithyre. They suspended themselves high above the earth, but had little power left to defend themselves for a very long time, until their numbers could grow again. For their own survival, they'd have to remain insular and secretive. Hence, not many on Áereth know about them.
But of course there'd still be rumors. Strange sightings of elf-like angels, pieces of their floating keeps breaking off now and then*? How odd would it be to find a castle spire in the middle of one of the Lostland deserts? Most folks would assume the sands parted to reveal a piece of a ruin beneath the earth...no one thinks to look
up. Maybe there's even an aerial/Áereth take on Area 51. A fortress somewhere on the ground that serves an the aerial elves' enclave to the ground world. Very hush hush, of course.
*Maybe
that's the current crisis. Maybe the magic's failing, and so the aerial elves are hiring adventurers (the PCs, of course!) or going themselves (if the PCs are aerial elves) to find the old lore to keep their castles afloat. Too much was lost back in year 0. Most likely the secrets of that ancient magic is buried far beneath the waves of the Lirean Sea.
So either the elves find the ancient magic lost with the fall of Ahna-Vithyre, or they have to bargain with or steal from the cloud giants, who've been doing the floating-above-Áereth for a lot longer. Either way, it's a race against time. Eventually those flying elf fortresses are going to come crashing down!
And that's all not counting the sphinxes. There's a lot you can do with them.