While I'm not particularly against Viera aesthetically, I wouldn't want them around: we alredy have a race to tick the "human with animal features" checkbox. Thematically speaking, you will have two races being too similar to each other. It would be okay if the theme of Eorzea was "people with animal traits", but that isn't the case (as evidenced by every other race, which are also quite distinct from each other).

I'd prefer a playable beast-tribe to have some greater visual variety in characters. It's a bit hard to come with anything else, though, because all the archetypes are already covered by the vanilla races: Roes are the big, strong guys, Lalas are the cute department personified, Elezen are tall and slender, humans are basic and Miqo'te are the animal feature race.

Actually, scratch that. You know what we don't have? DWARVES. Like, proper dwarves with very long beards. And no, lalas don't count: they are more like gnomes rather than proper dwarves.