Season 6 AU where after Tara dies and Willow saves Buffy but before she goes after Warren, she finds the talisman D'Hoffryn gave her in s4 and she decides in the heat of the moment to become a vengeance demon.
Vengeance demon rules make it impossible for Willow to wreak personal vengeance on the trio, not that Willow knows this until it's too late, but thankfully there is someone she knows who has no qualms wishing Warren dead, as we see in canon... Dawn. Not sure what consequences this would have but I personally would like to see it!
Becoming a demon also means all the pain and hatred and desire for vengeance that were too big for her frail little human body are now easily accommodated in her brand new immortal demon body, so Willow is no longer out of control. She’s in pain, yes, but this body was made to hold and nurture vengeance, so it cannot take her over or overwhelm her.
And she has Anyanka right there to show her the ropes and welcome her into demonhood! Anyanka would be able to tell that Willow is no longer human as soon as it happened, and now Xander and Buffy have to deal with Willow being a demon with no soul who is also still recognisably Willow, who still loves them and wants to fight by their side. Can Willow keep it up? Can she keep D'Hoffryn and Buffy happy? Anya struggled with it and eventually chose to give up demonhood but what if together they found a way to walk the line? I'd love to see a season 7 where Buffy has two vengeance demons on her side!
Wait, actually, vengeance demons can create whole new realities, what if instead of wishing Warren dead, Dawn had the foresight of thinking beyond vengeance and to the wealth of possibilities enough to wish for something even better? What if she wished Tara had never died? Could Willow grant it? Would they need to figure out a way to phrase it so they get Tara back while also wreaking vengeance? Or would they need to make Tara be the vengeance in order for a vengeance demon to be able to grant the wish? (Could they make her life vengeance against someone who hates her? Tara being happy and out and free could be considered vengeance against her family maybe?) What if the way they phrase it means that Tara comes back wrong? Or that they get a different Tara from another universe?