Hey there.
As we are already allowed using the transport book inside our homes for a while now, I think it would be a great quality of life feature to also allow its use in the guild castle, as well as the use of the home sweet home and guild castle transportation runes in each guild castle and home zones.
Additionally enableing the use of the "Black Codex Transportation Spell" skill in those zones would be great. Like that we do not have to leave respective zones to be able to travel to the others, saving some loading screens and time (and some crashes) while for example preparing for ini runs.
I dont know of any negative impact changes these would have, but if you can think of any then please share. 
Cheers!
