I’m surprised this isn’t already requested. It’s similar to Convert select fields into regular databases, but not the same.
I would love to be able to promote an existing single- or multi-select field to its own database within a space. I often find myself creating a space schema (all the different databases in a space), using a single select field, then later, wishing I could turn it into its own database to be related between other existing databases. The current workaround is to create a new database/type, then copy and paste the names and existing selections from the existing database, and re-create the relation. Since the select fields are secretly just databases, I figure this might not be that difficult to add.