Implement serialization of GUI state
At this time, GUIs do properly work – but only on the server.
That is, any external actions, e.g. calling lifts, activating teleporters, opening doors, ... are properly run, but the client GUI does not reflect the change in GUI state.
As with other features, e.g. carried weapons, the problem is drawing the line between predicted features, eye candy, and server world state. The GUI state could entirely be serialized, but it is questionable how that agrees e.g. with interpolations for colors or elements that change their size or angle of rotation, etc.
(Implementations might be implemented entirely passively, adjusting to the current server target value at any given rate, similar to other interpolation e.g. for NPC origins.)