Add a way for the ATC to join as a ghost, this is big for two reasons.
It would allow the expert ATC to join the game, and set up the ATIS in peace, without any stress of having to process aircraft. It would also allow us to become accustomed to the current state of the airport.
A feature could be added where if there is a controller on Ghost Mode, instead of the active controller leaving to give up a frequency, they can just press a button to transfer it to the current Ghost.
I think I heard from @CitrusAviatrix (all credits goes to her) of the idea of 2 controllers opening the same frequency with one controller being able to override the other, which may be used for the purposes of radar training. This should make things more similar to IRL training where the controller would observe the other.
Not sure how feasible that will be, but could be a step in this direction.
I would love to see a different UI setup to allow for this. Being able to swap controllers without fully closing, waiting the few seconds to minutes for the game to update, then jumping in, changing atis, and begin controlling allows pilots a few minutes to go crazy.
Since airports in the Expert server are usually busy, robot ATCs such as artificial intelligence or show can give wrong commands to pilots, pilots still do not listen to ATC because there is no violation anyway.