Breaking down some of the screenshots

[Krausus]

With regards to empty slots I have a theory. They had mentioned macros based on targeting the tank etc. Perhaps you can have pre loaded party slots and you move the characters into which roles they are playing.

Purely as example with a 4 man group, DRG DRG WAR WHM. Giving the option to put the WAR in tank slot and DRG as DD 1 and 2 would allow the WHM to have a pre made macro set and switch the party around to suit. Perhaps in the image with the empty slots these 2 roles were not present but remained there for if anyones joins.

This way you could prepare for different battles. On a tough fight perhaps you need 2 tanks so would could be designated T1 and T2 allowing all party members, esp the healers, to be much more prepared before a battle with much more effective macros.

Im guessing you can just move members around to where you want them