The Star Wars fandom has a PhD in speculation and theories and we’ve certainly continued in that fine tradition with Rebels. One theory that’s come up a few times especially in the wake of the Season 1 finale is that it’d be interesting to see Barriss Offee resurface as an Inquisitor. Personally? I think that’s a bad idea.
I’ve said more than a few times that I strongly disagree with how Barriss was used in The Clone Wars Season Five finale. I don’t think it made sense for her character and didn’t have the intended impact she’d rarely been in the show over the past few seasons. A different character could have better served the role of betraying Ahsoka. If you’re going to mess with a character to her very core than at least earn it through a well built arc.
However, this isn’t about The Clone Wars and what happened is canon. Although she fell to the dark side (I find this debatable) and blamed the Jedi for the war, we don’t actually know what happened to her. A good guess is that she was put in the Jedi’s prison basement which they (like the X-men and Avengers) totally have and after that, your guess is as good as mine. For all we know, Barriss was redeemed or maybe she was another casualty of Order 66. That said, I find it unlikely that she survived the Jedi Purge.
But for the sake of argument, let’s say she did survive.