If it were to happen, I'd have it happen like this:
Tara begs Osiris for Willow's life and Osiris tells her that the only way to undo what has been done is to sacrifice Warren in exchange for Willow's soul. So Tara goes to the magic shop and begins researching the ingredients required for her to exchange Warren for Willow and thus begins the hunt for him. Warren would probably still go to Rack after Buffy survives the gunshot wound (this would require Buffy to be shot in a different part of the body because if I recall correctly, Buffy was about to die AGAIN had it not been for Willow) and be warned by Rack that Tara was out for him, but Warren would most likely underestimate Tara because, well, it's Tara.
Warren, after not being helped by Rack because he killed Willow would probably go to the Magic Shop to steal some supplies when LOOK WHO IS THERE, Tara! Tara would use her magic to bind Warren in mid-air a la Dark Willow and begin the ritual. Buffy and Xander would have then just found Dawn by Willow's body and probably be mourning, and as soon as their minds would turn to wondering if Tara knew about Willow, Willow would come back to life just as Tara finishes completing the ritual, leaving Warren dead.
Now, it could go either way to make Tara step it up to Dark Willow levels. Either something in the ritual goes awry and corrupts Tara's soul so that an evil spiritual being takes over her body to begin opening the Hellmouth or another demonic dimension and Willow has to bring Tara back OR, the spell "works" but Willow isn't as she was before (For Game of Thrones fans, this might sound familiar) causing Tara to lose it (I mean, she just took a human life and that is bound to mess with someone) and she decides to take on Osiris herself, opening a dimension that causes Sunnydale to fall apart and Willow astral projects to stop Tara, somehow. I mean, Willow is trapped in her body but who's to say she can't still use magic?
I mean, they're not as good or as built up as Willow's arc, but if I had it my way, that would be the basis of turning Tara dark and just having it refined a little more could really make the story pop. We don't know what happened with Tara after her mom died so they could cut in flashbacks of Tara doing some heinous things to show what she had to do to overcome her evil side and why she's so respectful of magic now because I find that typically, addicts who have overcome their addictions tend to be more "respectful" towards the thing they abused (hence why Willow was acting a little more like Tara in season 7)
edit: OR, could have Tara sucked into a Hell dimension and set up the basis for season 7 where they have to go into the hell dimension to get Tara back or Tara comes back in season 7 as an agent for the First. I am FULL of ideas.