I'd only be interested in a 'third clan' for each of the playable races if they're going to be distinct both physically and in terms of their lore. It'd take up a lot of development time, sure, but it'd be a good way to include highly desired options for customisation.

So far only Hyur have major physical differences between their clans. For the other race's clans it's a lot more subtle.

As a rough example they could add Ishgardian Elezen with more muscle (which would make sense given that their people are built on strength and hardship) to appease those like myself who prefer their elves to be big and beefy.

For Au Ra they could go with something a bit closer to the original design displayed in the early concept art.