Ventress survived the Clone Wars as was shown in the Dark Horse series. Of course that could be changed in the series proper. Ventress was originally designed and created to be Palpatine's apprentice in "Attack of the Clones" but Lucas changed his mind and went with Dooku instead.
Ahsoka could very well be kept alive as has been theorized in previous threads. Apparently they've finally worked out her "fate" now and had multiple potential endings. I doubt we will see any of the characters from The Clone Wars in the Sequel Trilogy myself, but one never knows.