Well, personally, I don't see the necessity to make them something you can pick from the newbie intro? Could be like an 'experienced players only' sort of thing, like Death Knights in WoW. Here the gate would be lesson investment into your primary class, which would be a 'natural' class in your tether?
Edit for clarity:
I just mean that if you join spirit side then you have to start off with a natural spirit class. You wouldn't be cut off from the mirrored classes, you'd just have to reach the normal mark for multiclassing like normal (tri virt/duo trans), and then as a second class you could pick up the mirrored class or whatever.
Are you guys going to tie anything directly into the vampire subrace, or will each new class introduced to shadow be able to be used as vampire/undead/living?
Probably not, I think one class (on each side) gated behind a subrace is more than enough.
There are untold strategies and combinations still being found all these years later just in interactions between classes on the same tether. There are entire pieces of kits that go unused and unexplored. I don't really think it's a cause for concern, personally.
@Tanixalthas: You're amazing. You keep giving me so much to incorporate into my character, furthering their development, and I'm so glad that you indulge a more muted character like my own. Thank you so much.