As was discussed briefly in-game, actually, most Achur come from Saisio, lots of Amarr from Amarr...
There is no accessible locales in EVE beyond the homeworlds. Listing off the top of my head we've got...Caille, Hueromont, Lenoika, Dam-Torsad...er...that's the only cities available.
Yeah, if I was completely new to EVE, I wouldn't dare make up my own place either.
As the one who instigated said discussion, I'll post now like I said I would:
I was just starting out with RP in EVE when I created Morwen at the release of Empyrean Age, and had been intending on having her be from Caldari Prime as an instigation for her to eventually join the FDU. That plan got scrapped when I decided I was having enough fun doing regular missions in highsec, and after spending some time looking at PF, I realized I couldn't really find a lot of stuff about life planetside on Caldari Prime, so I switched her birthplace to Gallente Prime before it ever became a conversation point and I had to stick with it.
As for going with Caille in particular, that was mostly out of the convenience of there being some PF to work from, as well as not being able to remember the existence of Hueromont when I got around to specifying where Morwen grew up (
). I selected a district on the outskirts of Caille as an "it's a big cluster" and "yes, and..." measure. If there had been more PF about other planetside locations within the Federation at the time I was fleshing out some of these details, I'd probably have gone with something else.
As Seri said above, I pointed out that while a lot of Gallente RPers come from GP (or CP), and often Caille on top of that, it's the same with some of the other races (Amarrian RPers often come from Athra or Oris, many Khanid do Kihtaled/KP, a large number of Caldari RPers come from NCP or CP, and the majority of Achurans I've seen who make a point of noting their birthplace come from Saisio. I don't see it as often with Minmatar, but that may be attributable to the fact that Minmatar have a lot more "obvious" or PF-rich options available to them on top of the standard "born in the Republic" or "freed/escaped slave", including the Ammatar and to some extent, the Thukker.
I have a couple more things about Morwen's backstory that I'd like to flesh out at some point, but I've been a little too busy lately to get around to actually doing it.
EDIT: Also, kinda in retrospect, Caille being a cultural center for the Federation fits very well with Morwen's interest in fine/performing arts.