Trust me, "Rin might be a jinchuuriki" wouldn't even come close to the worst of all fan-theories.
Why do two of those sound like plots for fanfics? But I agree with you, Rin being a Jinchuuriki wouldn't be the worst fan theory out there. We have the half the Fox Chakra being sealed in Sasuke theory, (because that makes so much sense 'This Kyuubi is being controlled be a Uchiha, So i'll seal the other half in this few month old Uchiha baby, i'm sure his parents won't mind') then we had the Danzo is Obito or Madara's younger brother (the one he killed to get the EMS). Tsunade is Naruto's Grandmother, (So a teenage mother.)And ofcourse every red head introduced in Shipuuden is related to Naruto, (Though i'll admit I like that one, as I want to see Naruto having some family.)
But alas unlikely because I'm sure I read somewhere the First Hokage gave the eight Bijuu's to the other villages as part of a peace treaty or something like that, but that could be a fan created theory and I wouldn't know.