Because of the way level-scaling works, the devs have clearly designed the new Jobs with low-level play in mind. Even though they may be tied to Ishgard right now, I think there are a lot of reasons to consider also having a new starter-area for players to experience them right out of the game (something which I'll delve into later).
First off, let's address the primary concerns: When and Where would this new area be?
To be honest, it's something I've considered for a long time now. Initially, I thought Mor Dhona, but then I started thinking Ishgard (or rather, Coerthas). Honestly, though, as nicely as those areas would tie into the expansion, a better choice seems a far-cry from either of those places.
I'm talking about Rothlyt Sound.
Although we've not yet visited this small isle off the cost of Eorzea's eastern border, it makes more than enough sense to be someplace that the Au Ra would set up camp. Although the isle itself is small in comparison to Eorzea proper, it's still large enough that you could easily include enough content to get players from level 1-20, until they could get to the main continent, where the story merges anyways.
Whatever Rothlyt Sound might have been before, now it would play home to the first Auri settlement. Because the Xaela are nomadic by nature, they would likely make up the bulk of this first Doman scouting-party. As opposed to the other City-States, which are very much "Capital Cities", this new "City-State" would be much more primitive, emblematic of the Au Ra's plight as outcasts. They would have more barebones, tribal-looking structures, but just like their starting clothes, their tents would be adorned with various trinkets, and some with bones.
I imagine Rothlyt Sound's environments probably aren't totally unlike La Noscea. Very tropical climates, but maybe more jungle canopies, or even mountainous. Really, we know hardly anything of the area, and since it would only serve the purpose of getting us from level 1 to level 15-20, it gives the artists quite a bit of freedom, without necessitating a ton of resources. Aside from Auri architecture, a lot of assets of Lo Noscea could probably be reused.
Barring another name, I like the sound of "Warrior's Rest" for this starter city. It's simple, but powerful, something that I think would align with the Au Ra's values, the Xaela in particular. Here, the Xaela would instruct newcomers in the ways of the Dark Knight, whose teachings have been passed down since they first learned them from an "ancient deity" (referencing Odin/Zantetsuken). The Raen, meanwhile, would teach others to commune with the stars as Astrologians, something they've been doing for as far back as their stories go (perhaps having first learned the technique from Ishardians, in the far-distant past). There would also, however, be a few Garlean defectors, who are helping the Au Ra "fight fire with fire", by way of the Machinist. I would probably also consider adding the Rogue here, available for new characters, but that's just me.
The story would bleed directly into the original stories, with you joining the Grand Companies. Really, the only story-beats you need concern yourself with are meaning at agent of the Scions of the Seventh Dawn, which I needn't think to be a massive issue. If anything, it might be a great opportunity to introduce a new character.
Ultimately, this probably sounds way more ambitious than it would actual be; you would still be hitting all the same plot-points, it just helps cement the Au Ra into the game's Story and world, while also providing a new starting-experience for the new Jobs.