I'm not sure why people are debating Yugiri being a Gria as they are already in the game. E-Sumi-Yan, the leader of the Conjuror's Guild in Gridania is a Gria... the lore surrounding them also makes it highly unlikely that they will ever be a playable race.

As for the Viera... I think they're somewhat unlikely any time soon simply due to their being so similar to Miqo'te. We're more likely to see distinct races first and Yugiri's clear reptilian features certainly make a lot of sense in that regard.