It's really hard to say. I think, initially, when Korra was supposed to be a one season thing, it seems that they had it in mind that Everyone but Katara was gone (from the initial group that is, we know Zuko's still around from the "Republic City" game on the nick site). However, since it was never specifically stated, the writer's could always bring Toph back as a kick ass old lady if they really wanted too.
Katara: "Aang's time has passed. My brother and many of my friends are gone."
Katara made a lot of friends during the course of the series. Teo, Suki, Haru, The Duke, Longshot, most likely Ty Lee, etc. etc.
So again, even what Kataar says isn't exactly conclusive. While I'm under the impression that it's 90% likely that Toph isn't around anymore, it is, again, pretty much up to the whims of the writers as they haven't said anything conclusive about whether or not she is or is not alive anymore.