In a config flow it would often be nice to have some sort of dynamic fields. For example show / hide or activate / deactivate a text field based on the selection of an option list (SelectSelector).
Right now such things are solved by using multiple steps. Imho this is bad UX as a user can’t see required parameters at a glance when playing with options.
Here is an example of what this could look like:
Make a proposal for a format 
We have already something similar for service.yaml, based on supported features.
I guess we would pass it into description on the markers in voluptuous. That’s how we instruct ha-form to also provide dynamic data (ie. look at the first 3 fields, they are related)
I believe karwosts did something already but it hasn’t received any traction. Let’s see if I can find it.
I wonder if you might be thinking of this: https://github.com/home-assistant/architecture/discussions/1117. Not exactly the same as what’s being asked here, but some similarities.
Didn’t really start any work on it, but just starting a discussion about what kind of extensions we might be interested in.