Imagine that Naruto and Hinata get a kid. It is possible that the Hyuuga-clan descend from the elder son of the sage of six paths, and the Uzumaki-clan are distant relatives of the Senju-clan, and the Senju-clan descend from the younger son of the sage of six paths. To acquire the rinnegan one must have the DNA from both brothers. then, isn't it a high chance their kid would possess the rinnegan?
The exact lineage is still unknown. The only character to be preaching "We are the descendants of the elder brother with the Sage's eyes" is Madara (and Tobi, who got it from Madara). Kakashi, himself, stated that Madara's words should be taken with a grain of salt and conclusions should not be jumped to.
It's possible that things could get all kinds of wonky in the blood lines.
Then, there's also little logic behind the idea that a kid born to any 're-unification' of the blood line would, in fact, have the Rinnegan (or access to it).
Personally, aside from the ability to use certain OP jutsu (which seems to be about all it is... an excuse to DBZ the hell out of the plot)... the Rinnegan seems largely inferior to the Sharingan and Byakugan when it comes to visual prowess and tactical usefulness. I'd rather have the Byakugan, myself.
