Iymrith could haunt the surviving members of the Party after she and half the Adventurers were sucked into ... nobody knows where.
This involves a bit of re-writing history ... but we are in an Epic Fantasy world after all.
I'm thinking of something like the 4e Dreamwrought Dragon: you fall asleep and Iymrith taunts you (via a Dream/Nightmare spell) until you wake up in a cold sweat, not feeling very rested.
Iymrith is clever and subtle, she ought to have minions available to send against the surviving Adventurers - and their friends too?
If you ever form a new group, there is a dangling plot thread from Ye Olden Days for the new group to resolve.
After about 14 levels of being mocked and thwarted and threatened from 'just beyond reach', the new players ought to be gunning for Iymrith too.
DM's Eyes Only: the secret is to set off another dimensional rift in the same place; this either pulls the PCs into the Astral Plane (or wherever Iymrith has been all this time) or pulls her back to the amphitheater.
Give the PCs enough clues that they can prepare a way back this time. And Iymrith might conceive what Raistlin was going to do to that cleric-woman in the process of facing Dragonlance's 'Tiamat', should anybody be bold enough to track her down.