So, to the back to the OP a bit: we now know that the Vecna campaign will tie into and revisit Adventure sites from the past 10 years, could we be returning to Omu and so in, following these Obelisk locations?
Been reading up on the Spell Weavers, and boy howdy there is some potential there.
Apparently, there is one canonically "surviving" Spell Weaver, who had ascended to godhood in the FR: Jergal, the ancient God of death, who also coincidentally taught the Netherse how to magic, and played a significant role in Baldur's Gate III more recently.
Could he be an enemy of Vecna? Could Vecna have learned secrets and lichdom from Jergal? Ed Greenwood is on record that Jergal is the greatest puppet master villain of the FR.
Maybe the Spell Weavers built the Infinite Staircase, and it is related to the Obelisks?