Well, we have lots of guesses on to who it could be. The most common ones being McGucket, Stan, or Bill Cipher (hopefully NOT the last one)
But considering that Alex respended to "What will happen to Waddles at the end of the summer. Will he stay with Stan, or will he go to live with Mable at their parents." He responded that thinking about that made him cry, meaning the death might be:
Waddles
Stan
Dipper and Mable's parents
What do I think? I think that it will be Dipper and Mable's parents, and the twins will move in with Grunkle Stan from now on and stay in Gravity Falls (if they are going the Dipper X Pacifica route, wouldn't it make sence for the twins to stay in the town, as well as Mable having her two best friends)
What do you guys think?