Things have happened much as they did in the book. Asajj and Quinlan Vos worked together, she taught him the Dark Side as the Nightsisters understood it, they found love (of a sort), she lost him to Dooku and the Dark Side, then found him again, and then took Dooku's lightning strike to save Quinlan. Quinlan and the Jedi left her in the Nightsisters' Pool of Life, thinking she was dead and went on their merry way, patting each other's back for having lead another soul back to the path of the Light Side.
What happened next is where things go AU.
Asajj floating in the pool for who knows how long. The strange magics of the Nightsisters keeping her just barely alive and in a coma as the pool fought to heal her body. It succeeded. Asajj awoke and swam to the surface. It took sometime for her to regain her awareness and whom she was, but she did so.
More time passed and then a ship came to Dathomir, a ship piloted by Ahsoka. Feeling a pull in the Force, Ahsoka had no idea what she'd find once she got to Dathomir, but once she found Asajj, she just shakes her head and realizes she should've known. Ahsoka brings Asajj up to speed with all that has happened in the galaxy, turns out Asajj was out for years. Ahsoka tries to recruit Asajj to the growing Rebellion but Asajj has had enough of war and politely declines.
Still late, Asajj is working an odd bounty. A simple job really but one that pays decently. All she has to do is guard a building, allowing no entry; not even herself. She breaks this condition when she feels and hears someone in pain within. Finding a pregnant twi'lik who is about to give birth (with no medical assistance beyond an old droid), Asajj quickly discovers the whole building is filled with pregnant being and realizes her employer is up to some damned shady things. She can put up with a lot, but this is beyond her tolerance. She lays waste to her employers facility and henchmen, and flies off to parts unknown with the women, many of whom give birth in flight and no of whom want to return to known parts of the galaxy.
Eventually, they find a planet to settle that appears on no galaxy maps. It is something like Dathomir but less...sinister. Swamp-like, the entire planet is covered by one massive mangrove forest. There is no land, per se, but the tree are gigantic and provide needed terra firma. (Think Endor sized mangrove trees with interlocking branches.) They decide to settle there and Asajj is surprised to find herself wondering if they could become a new Sisterhood.
There is more to it, but that will have to wait until I write the fan fic.