There are some feature requests here that can be generalized by this feature:
Allow typed lists fields. That is, fields that are a list of values of some type. For example, multiple checkboxes, multiple urls, etc.
Currently just a few types accept multiple values: selects, attachments and links to other records. Why not make this more consistent: every type can be single or multiple, that is a restriction on cardinality orthogonal to the type itself.
I understand this is a bit at odds with modelling one to many relationships between tables, but it’s very convenient for simple fields (you already recognized it by providing multiple attachments and selects) and avoid having to cascade delete or removing orphans after deletion.