Zargon!
Zargon, baby, Zargon!!!
Like delericho (above), I was pretty sure I had bought my last 3.5e book... but it's within the realm of possibility that I'll buy Elder Evils for that one chapter alone. B4 - The Lost City is my favorite all-time module, and a really good treatment will be worth the price of entry.
I've run the entire Age of Worms campaign, but I'm still also moderately interested in Kyuss as well.
As for the rest...? I'll keep an open mind. Atropus is certainly... different. Some of the others sound like they're worth a read as well. It's a shame they didn't pull from a bit more established D&D fluff. I would've loved to have seen a treatment on, say, Kas (as in Sword of Kas... Kas and Vecna). Or Saint Kargoth the Betrayer (first of the death knights... or even Lord Soth). The return of Azalin (uber-lich from Ravenloft setting). The demigod (?) Arik, from B3 - Palace of the Silver Princess. Iuz the Old (too Greyhawk specific?). That nihilistic rubbish-heap from the Realms called Moander... or even the return of Bhaal (god of murder; who I feel is under-treated given his central role in the best-selling Baldur's Gate games).
So many options, just mining the classics...