Currently, an Indexed colour image can have just one layer.
I would like to see this mode extended to allow multiple layers, with one Color Table per layer.
The colour table could be indicated by a small multi-coloured icon either to the right of the Mask thumbnail (if there's room!), or indented and below the layer (much like layer effects).
By default, a newly created layer should inherit the Color Table of the active layer.
It should be possible to alt-drag a Color Table from one layer to another, which would have the same effect as if (now) you opened the Color Table dialog and loaded a custom Color Table from an .act file. Because this can introduce such radical changes to colour, Alt-dragging a Color Table should throw up a warning/confirmation dialog.
It should also be possible to lock the Color Table for a layer.
This suggestion is related to the suggestions in the thread Color Table pane and enhancements. (A Color Table pane would be a practical necessity if Color Tables became available per layer.)