Are we able to customize Origin characters appearance and subclass when playing them? Like changing Shadowheart off Trickery domain or Wyll to Great Old One?
I'm afraid I didn't note this for definite, but I don't think we can. In fact, I think someone shared a screenshot on these forums somewhere showing the Edit Appearance option was greyed out for origins other than The Dark Urge, and from admittedly hazy recollection we simply don't have a class/subclass tab for them.
Definitely feel free to wait for more reliable and better evidenced info from elsewhere, but I'd brace to find that the non-Urge origins are pretty fixed.
"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"