More dynamic field properties via functions

[DieTapete]

Hi there,
the config definition of a collection has the option to show or hide a field via the admin.condition function. However if I want to change an arbitrary field's property dynamically the only option is through custom components or plugins, which is quite cumbersome.
I have currently two use cases:

1. I want to set the description of a field based on another field's value
2. I want to make a field readonly after the document was created.

My proposal is to always add the option to use a function for setting a value:

description: ({ subtype }) => ['subtype1', 'subtype4', 'subtype6'].includes(subtype) ? 'Description 1' : 'Description 2',
readOnly: ({id})=>Boolean(id)

To my mind this would add a great value. However, I just got started using payload and I guess that the feasibility of this approach might be limited.