As mentioned earlier, the Japanese letter specifically referred to the new tier as "Zodiac Weapons".

However, Yoshida spoke of it as a "new concept", so hopefully that means it's going to be a different upgrade mechanic than going from relic to zenith.

The name alludes to the job specialization system used in FFXII, and some Japanese forums are guessing that it may allow players to improve their zeniths through customize-able upgrades. Or that it is a completely separate tier running parallel/above relics.

Remember that "Zenith" means "the highest point" in English, and they went through all that trouble changing the +1 to Zenith. It wouldn't make a lot of sense for the new weapon tier to be another simple upgrade.