In the same interview where Yoshida reveals that there will be a new relic-type quest that is separate from the current one (i.e. does NOT require the current one to start) in 3.1, he also reveals that those who complete their 2.0 one will be "taken into consideration"
It's about as vague an answer as you can get, but considering the damned things aren't scheduled til 3.1, which is likely a good 2-3 months after 3.0 launched, they probably don't even have the details hammered out.
In fact the language he uses when asked is "OF COURSE!"
So yeah, we're getting something in 3.1 if we have our full zodiac.
I'd imagine, based on how the Relic->Zodiac quest went, that the 2.51 zodiac will be able to be used as a catalyst for the next one in the same way the Nexus was for the Zodiac, thus allowing us to skip a few steps much like the ones joked about during the quest itself.
