It would be extremely useful to have a type of custom field (or a field behavior option) that allows new values to be appended to the existing content, instead of overwriting or clearing it.

Let’s say a contact initially enters "Milan" in the City field. Later, the same contact interacts again and provides "Rome". Instead of replacing "Milan", I’d like the field to automatically update and display something like: "Milan, Rome" — keeping a history of values or indicating multiple relevant entries.