My theory is that it'll be Old Man McGucket. Why? I'll show you.

In the season two promo image, we see two hats on the ground beside Dipper and Mabel. One is Stan's, and the other is McGucket's. I think this signifies that these characters have very dark futures. However, I don't think Stan's going to die. It makes more sense for him to give himself up in an act of selfless sacrifice, but live. He is way too important a character to die.

McGucket, however, is not. He has only had really important roles in a couple of episodes, but is still capable of making that sacrifice, but this time it doesn't end well. His son may finally come to grips with how much he really loves his dad, and will receive a hero's funeral at the end.

So what do you guys think? Is my theory plausible? Leave your own theories down in the comments!