Board Thread:General Discussion/@comment-3434597-20150302063319/@comment-5475153-20150402015210

One might suspect Dipper and/or Mabel, if there isn't a third season. Alex Hirsch's latest interview gives the rhetorical quote: "How are the children’s destinies tied up in all this [the events of the series]?"

- Alex Hirsch

Destiny may imply some sort of finality, and if the twins are achieving their destinies at such a young age, it might make sense for their lives to be cut short.

Of course, this may be a bit too dark, so my other guess would be Stan.

In any case, the appropriate device will probably be either the classic "heroic sacrifice" or the ultimate downfall of a major villain.