I’m probably never going to get over how much Dracthyr missed the mark (especially since I actually love the gameplay of the Evoker), as the total lack of transmog support really hampers the fun of playing the game.
Generally, I would consider any transmog limitations to be a complete deal-breaker for pretty much any new race to be added to the game.
The sole exception to that would be Naga.
Now, in saying that, they obviously would need to be able to wear armor on the upper-body. Their prominent features (fins?) on their head would also prevent most helms from being moggable, although I would probably suggest adding a selection of “Special” helms to be used (ie. holiday items like the Winter’s Veil hats, or the promotional helms, maybe a smattering of “classic” Tier set helms).
I don’t think anyone would really balk at limitations there, because their physiology literally prevents them from wearing most helmets, or any pants or boots.
One thing I would probably consider is for their tail, maybe having a “bangle” that you can transmog your bracers to. My rationale is that Bracers are uniquely designed to look that way, and there are a ton of textures to match virtually every Tier set in the game, giving them a ton of transmogging potential, not just their torso. Maybe you could have different “styles” of bangle at the Barbershop, so even though it’s ultimately just a simple texture being stretched over it, you could still play around to find a design that compliments your overall transmog.
While I wouldn’t consider it strictly necessary, I could also see maybe having a Lore development that perhaps Azshara (or someone else…?) grant them the ability to give themselves legs again? I don’t think it’s the craziest idea in the world; similar to Trolls, I could totally imagine them having clawed feet that don’t display boots, but still being able to cover their lower legs.
Again, just an idea. I’ve always liked the idea of more “exotic”, less “human” races (Draenei and Tauren are basically the only ones), and Naga have a long established, and fairly well liked presence in the game and the Lore already.
Like I said, I love me some transmog, but that’s probably the one and only race I would be happy to give up a little flexibility in order to see added.