Two minor issues with the spectre dimension:
1.) If you loose your key, you can't exit the dimension.
2.) You can exit the area via any teleportation method from other mods

Suggestions:
1.) Allow the player to exit by right-clicking the spectre core (possibly holding right click or standing still for a while after clicking?)
2.) Have the surrounding area automatically teleport the player back (as if they had used the Spectre Key again) to prevent teleporting out

1 Is a great idea! :D

2 Should already be the case, when you exit the cube you should be teleported back (Unless you are in creative mode)

2. Hrm. Must've had creative mode on. Thanks for the info! :-)