Right so in the last chapter we've seen some of what the royal palace area looks like and we've been told about the 5 cities each Royal Guard member "owns" and it got me thinking like...

When Hikifune got promoted to the Royal Guard.. does that mean someone left? I really doubt they'd make a totally new platform-city just for her and it seems more like the Royal Guard always consisted out of 5 members for each of the cities.

So who else could it be besides Isshin?