There are extra dimensional and scientific references strewn about the entire first season. Considering the scientific basis for Gravity Falls, and the sheer originality of the writing, I am making the prediction that a "twin paradox" will be featured.
A twin paradox is a theoretical principle where one twin travels away from Earth at light speed, then back. Due to the nature of space-time and relativity, time itself slows down for the twin in transit while the twin on the planet continues to age.
If Stanley Pines exists and left Earth through the portal, then began a return journey in a space craft. It would be possible for him to return, having not aged much and re-unite with his brother Stanford, who has aged considerably.
This is the principle behind The Twin Paradox. Two identical siblings who have aged at different rates.
Knowing what we know about Gravity Falls, I suspect this story potential to be irresistible to the writing crew and fully expect to see it played out in future episodes.
I realize this is a radical assumption. It is also an educated guess.
Please share your thoughts on the subject below.