The Man From Earth/WMG

The Man In The 1600's was...

 * Another of John's relations, it's not too much of a stretch to guess that the Immortal Procreation Clause got averted properly for at least one of them.
 * Kenneth from Thrity Rock.
 * Vandal Savage. (Inspired by a quip in the Expy entry in the main page.)
 * The Doctor. He just happened to see the same incarnation two times a few centuries apart.

John is Mr. Flint

 * or at least an alternative version of Mr. Flint, as Star Trek was mentioned.
 * John is 14,000 years old, more than twice as old as Flint claimed to be. Maybe the other immortal John met in the 1600's was Flint.

John is The Emperor of Mankind

 * A bit of a stretch, perhaps, but his people did think he was a shaman (psyker) and the Emperor is at one point implied to have been the inspiration for Jesus. The main problem with this (besides the otherwise vastly different tones) is that John claims to be (probably) from (what is now) France, whereas the Emperor is from Anatolia (now known as Turkey).

John is just a delusional regular guy after all

 * But he has the same father as Gruber. The guy has a habit of abandoning families and leaving messed up kids behind. John knows stuff from Gruber's childhood because his father told him, and looks enough like his father to provoke the shock of recognition in Gruber who last saw the guy some 50 years ago.