These are all just thoughts and theories, but there are some spoilers if you haven't seen the last few episodes of Gravity Falls.
- Since the brother looks exactly like Stan, he could (aside from the obvious twin) be Stan from another universe, or a clone, which could explain why he has 6 fingers.
- His brother, in going through the portal, could have ended up in an alternate/parallel universe to gather the knowledge the book spoke of.
- He could have been dead. (Lame theory)
- Or traveled to a different time, and that's why the portal had to open at a certain time, for it to connect correctly. Maybe the brother traveled to the future, and had something to do with the time paradox avoidance enforcement squadron.
- This could have been the project McGucket and the brothers worked on, researching and creating the portal to gain the knowledge. edit: But wait a minute, if they're just researching it and building it and have never done it before, who wrote all the warning labels in invisible ink? Did the brother? Does that mean he fired it up before he went through it? And if so, did he lose someone in it or destroy the universe or disrupt time??
- I wonder if the brother and Bill know/worked with each other. Can Bill travel to wherever the brother was? Did he make a deal and that's how they were able to research/create the portal? edit: And if so, maybe that could tie into the last edited theory, that if he lost someone through the portal, maybe it was because Bill screwed him over (like he does).
- If the brothers knew of the creatures, how did they escape the Eye group while researching them? Did they create the group, since you see Mcgrucket with the gun, and it was later always in the group's cave? Did they try to work with any of the creatures or keep any of them in the house?
- And I know everyone's thinking it, but it's still a good theory: the brother is the villain of this season. Or a major dick. Or both. I can see him being a complete jerk to Grunkle, like pushing him away during his research, and criticizing him, or badmouthing him in front of the Pines twins.
Listening to: Cog is Dead, Amon Amarth
Reading: Natsume's Book of Friends
Watching: MASH, Friends, GF
Playing: Nevermind demo