Learning that FFX and FFVII are in the same continuity certainly added some layers of speculation to Jenova's origins. While existence as an aeon seems unlikely as there are no more aeons or Yunalesca, there are remarkable similarities between Jenova and Sin (a large fiend pulled together with Gravity spells).
Both seem to have a thing for tentacles, both are associated with Time/Space magic (Jenova with Meteor, Sin with Gravity), both can metamorphose, both spawn off smaller creatures from dismembered portions of their bodies, both can affect memories (in Sin's case, this is due not so much to a deliberate thought, but to the gravity field around the creature constantly sucking up pyreflies -- even to the point of tugging on those within a person's body), and both eventually reunite with separated parts of their bodies (Jenova has her Reunion, while Sin is known to return to reclaim its Sinspawn).
Given that we know from FFIX and The Spirits Within that the Lifestreams of different worlds may be poisonous to one another as one seeks to convert the other, Jenova's influence -- and, thus, Sephiroth's -- on the Lifestream of VII's world may be in that same vein (e.g. turning the Cetra into monsters, corrupting the Lifestream, etc.). At any rate, it's not difficult to imagine that Jenova's existence began in a similar fashion to Sin's.
