The continuity police are all over the fact that Ahsoka doesn’t appear in Episode III, nor is she mentioned in the film. The fact that the events in Episode III happen in fairly quick succession, makes it possible that she simply didn’t come up in the discussions between the characters, but what if there is another reason. What if Ahsoka is not mentioned because the other characters are still dealing with her death and are aware of how emotionally affected by the death Anakin was. Members of the Order and Padme would be extra sensitive to Anakin’s emotion because in the case of the Jedi, he is viewed as the Chosen One, but he is also viewed as having attachment issues that could be a path to the Dark Side. As such the Jedi Masters would be wary of wanting to push him for fear that he could fall. Padme may not bring it up out of love, because she knows how much the death affected her husband.
We have a number of characters that could potentially bring about the death of Ahsoka. I don’t think they would have a minor character kill Ahsoka, and I also think it is more likely then not she would be killed by a Force-user The most likely of which would be in no particular order, Count Dooku, Asajj Ventress, Savage Opress, and Darth Maul.
Darth Maul is set to make a return to the realm of the living in TCW, presumably we will see him at some point in Season 4. His return was foreshadowed in the Season 3 Nightsisters Trilogy. Dual-bladed lightsaber versus Ahsoka and her two lightsaber fighting style would be a real cool battle to see. It would also set up the scenario where Anakin kills the resurrected Maul out of revenge or hatred and furthers his inevitable fall to the Dark Side. The Savage Opress killing Ahsoka story would be roughly the same as the Darth Maul one.
Asajj Ventress began the series as one of the main villains and even though she has broken with Dooku, perhaps she could hunt and kill Ahsoka in a bid to get back into Dooku’s good graces after the Nightsisters, potentially her only ally are defeated.
Count Dooku of the villains would be my favorite candidate to kill Ahsoka. Lets take a look back at Episode III, when Anakin kills Dooku.
Supreme Chancellor: [Anakin cuts off Dooku’s hands ending the battle. Anakin catches Dooku’s lightsabre and ignites it and puts both lightsabres to his neck] Good, Anakin, good. Kill him.
[Dooku looks at Palpatine in shock]
Supreme Chancellor: Kill him now.
Anakin Skywalker: I shouldn’t…
Supreme Chancellor: Do it!
Anakin Skywalker: [Anakin decapitates Dooku] I couldn’t stop myself.
Supreme Chancellor: You did well, Anakin. He was too dangerous to be kept alive.
Anakin Skywalker: Yes, but he was an unarmed prisoner.
[frees Chancellor Palpatine]
Anakin Skywalker: I shouldn’t have done that. It’s not the Jedi way.
~IMDB Ep. III Quotes
Why might Dooku have been to dangerous to be kept alive, why might Anakin have been angry enough with him to kill him in cold blood? Perhaps Dooku killed Ahsoka in the waning days of the Clone Wars.
One potential odd ball fate is related to the Mortis trilogy in which Ahsoka died and was brought back to life by the life force of the Daughter channeled into her by Anakin. Perhaps this fix was only temporary and Ahsoka eventually weakens and dies as the Daughter’s life force fades within her.