I want to preface this by stating that I've always venomously disagreed with the retcon that merged Tasha and Iggwilv into one person. In my games they are two separate people, with two different personalities and goals. Tasha is a whimsical fey mage, while Iggwilv is the vile demon loving witch queen of lore. Even if you love the retcon, you could still accept the premise of using an Iggwilv avatar instead of a Tasha one for the time-travel shenanigans.
I'm planning on running the adventure, but I'm already going to modify the beginning to be Greyhawk instead of the Forgotten Realms (using Dyvers instead of Neverwinter). So, what if we replace Tasha in the adventure with Iggwilv? I feel that Iggwilv is the more interesting of the two, and she's also the aspect most relevant currently in my campaign. Additionally, I enjoy the symmetry of having each alignment represented by the Mages Three, with Alustrial representing the forces of Good, Iggwilv the forces of Evil, and Mordenkainen representing the Balance. Despite being evil, she will still want to be part of stopping Vecna, because she sees him as a threat to her own plans.
What issues can people foresee? The only one I can think of, is that Iggwilv's not going to be cool with Mordenkainen's plan of dropping Vecna back in Greyhawk (Tasha should also have an issue with this, but somehow doesn't). I'm already working on an extra chapter where the party has to stop Vecna back in Greyhawk for the final time, so I should already have this issue addressed by the time we start.
I'm planning on running the adventure, but I'm already going to modify the beginning to be Greyhawk instead of the Forgotten Realms (using Dyvers instead of Neverwinter). So, what if we replace Tasha in the adventure with Iggwilv? I feel that Iggwilv is the more interesting of the two, and she's also the aspect most relevant currently in my campaign. Additionally, I enjoy the symmetry of having each alignment represented by the Mages Three, with Alustrial representing the forces of Good, Iggwilv the forces of Evil, and Mordenkainen representing the Balance. Despite being evil, she will still want to be part of stopping Vecna, because she sees him as a threat to her own plans.
What issues can people foresee? The only one I can think of, is that Iggwilv's not going to be cool with Mordenkainen's plan of dropping Vecna back in Greyhawk (Tasha should also have an issue with this, but somehow doesn't). I'm already working on an extra chapter where the party has to stop Vecna back in Greyhawk for the final time, so I should already have this issue addressed by the time we start.