Allow all fields to be "multiple"?

[recidive]

The questions that pop up whenever I work on field extension is that if we should allow every field type to be multiple.

For now we allow e.g. text fields to be multiple by creating a non standalone sub type and a field to reference that on the main type, but since we allow fields to use other elements, how would we handle e.g. the case when we want multiple options to be chosen from a short list without having to create a sub type and a few items either on a hook or through a few JSON files? That would be awkward on this very case.