I think the Custom field types card on the Atlassian platform for developers roadmap covers this:
A custom field type is for a custom field what a class is for an object in Java (or most other object-oriented languages). Or, in other words, it’s a template from which many actual fields can be created. You can have one custom field type, and many different custom fields of that type.
We will create a new module type for apps that they will use to declare new custom field types. It would look something like this in the manifest:
modules:
jira:customFieldType:
key: scripted-custom-field
name: Scripted Field
descrition: A custom field showing data returned by a script
configuration:
function: configureField
view:
function: renderView
You can watch the card for updates, though please note that there are only few comms on the cards and they are not always synched with status changes in a timely fashion - I still consider the board a highly valuable overview of the Atlassian team’s current priorities and activities, and it also gets better over time, see Enhancements to our ecosystem roadmap for developers!.
Turns out you are in luck @martin.gypser and that has been a sneak peek indeed, today’s Forge release 2021-05-04 delivers jira:customFieldType as a beta module (similar to the jira:customField being beta still as well).
The jira:customFieldType link from the changelog has no target yet, i.e. the docs have not been published, but you can already explore this on your own by means of the resp. jira-custom-field-type-ui-kit app template