The UNION list operation is very useful to combine two lists of the same entity type. It would be very nice if it works for inherited types as well.
Say that you've got a Notification entity and inherited types based on this Notification entity. E.g. TeamNotification, UserNotification and a bunch of others. If you want to get the list of TeamNotifications and the list of UserNotifications together, you can't combine it with the UNION option.
In my ideal world it should be possible because they are from the same base type. The result of such an UNION is a list from the type of the first base entity they share. In this case the Notification entity.