From gamerescape interview.

http://gamerescape.com/2015/03/09/pax-east-interview-with-naoki-yoshida/

We first asked about item progression in Heavensward, and how completed Zodiac weapons or Lucis tools would be addressed. Yoshida first confirmed that there would be new “relic” progression within the 3.x expansion. It is not yet finalized whether it will be identified as a continuation of the 2.x progression or a brand new concept, but it will continue throughout the 3.x expansion, starting at 3.1. For those who have completed Zodiac weapons, they are planning on having some advantage for those players in this new progression. Yoshida mentioned that players should understand, however, since the level cap will be increased to level 60 and with higher item levels, there will of course be new weapons with higher item levels. The new “relic” arc will start at 3.1. We also confirmed that level 51 gear will start below iLevel 135.
This makes it sound like there will be 1 relic quest line it will start in 3.1 if you have a Zodiac you get an advantage over someone who doesn't have one, maybe skipping first couple of grind steps.