This is a really good question to be honest . I'd have to go with Somewhere far in another dimension and it can only be revealed after a certain something happens such as the earth or SS being destroyed.
** On a side note i'm going to go out on a limb and guess that Ichigo is the soul king
Would be funny if aizen got the key, and it didnt fit.
anyway yea i think that the realm is not as big as SS but just like the size of seireitei were all the captain level shinigamis who live there could fit in like nothing and the king is probably like some spirit or something that cant fight unless someone wield him, i assume he is like a spirit of a zanpakutou but thats just mi opinion u know like shaman king's great spirit
I wonder if when aizen uses the key, it'll look like this:
I think the kings realm would look like a Shogun's castle or maybe Kubo would completely change the style and go for that europian aestetic for the setting, and the character designs. But that'll be too ironic even for Bleach.
Also, stop assuming that the Royal Guard are on an awesome 10 times stronger level than normal captains; we don't know how strong they are. Plus, if they've seen Aizen's shikai, it's game over regardless. He can manipulate even their sensing of reiatsu, because he puts them under complete HYPNOSIS, not illusions.
Blergh, old thread is old, I realized I just posted the same refutation twice... oh well.