There was a deleted scene from ROTS where Grievous kills her on board the Invisible Hand in front of Anakin and Obi-Wan, but that was scrapped, so TFU probably saw it as okay to rewrite her death. As of right now, all we know of how she dies is that she's impaled by a blue lightsaber (Yoda saw this in a vision in his last arc on TCW), which leaves both scenarios as possibilities, I suppose.
Anyway, back on the Rebels front, I've thought a lot about the finale in the past day or so, and re-watched the important scenes multiple times. I am now almost completely convinced that Ahsoka is alive. The shot of her walking into the temple clearly has her descending, as if down stairs, rather than collapsing. This is supported by the T-Shirt Ashley Eckstein has been shown wearing, which clearly shows Ahsoka walking into the temple "unharmed" (I put that in quotes because we can't know that she hasn't sustained injuries). I don't know if she'll be back next season, but I'd like her fate to remain unanswered for a while. There is a novel coming out about her adventures between leaving the Order and joining the crew of the Ghost, so that'll provide us with plenty of Ahsoka action in the interim.
Now, as for those who say that leaving Ahsoka alive invalidates Vader as a threat, I respectfully disagree. For one, that explosion was pretty rough on the temple. I'd be surprised if Vader and Ahsoka even had much time to duel before it collapsed, and the state he was in after it fell isn't exactly one where he should be fighting. Perhaps they were separated during the blast? Maybe Vader allowed her to live in the hopes that she would lead him to Kenobi, or turn to the Dark Side?
Certainly a lot to think about...