Currently, label classes in multiple dialogs / interface elements of both ArcMap and Pro are totally unsorted and listed in the order in which they have been created. This is problematic for layers having large numbers of label classes, and makes management of the label classes difficult. It would be really helpful if there was an option to (automatically or manually) sort the label classes, and have this sorting persisted when an map document or layer is saved.
If full automatic alphabetic sorting is used, then persistence is probably not necessary and likely easier to implement for ESRI, although I can imagine some people would like to have custom manual sorting as well.