Would be great to have this feature considered, if it wasn't already mentioned:
Say I have a custom field called "source" of the contact. It's a multi option field. I choose "referral". Based on that, it would make visible for entry additional dependent fields. Otherwise those dependent additional fields would be invisible or marked not visible. So if it wasn't set up to always be visible, it would actually only become visible if that selection is made.
This allows for a more dynamic experience and doesn't end up listing every possible field in the detail section, taking up space with empty fields when they aren't necessary yet.
Having them hidden and manually adding them to be visible (by expanding or collapsing fields) is what we do today. But having this smart dependency feature would be great.
How I'd implement it if I was a tech at PD?
For each field just add a section when you create a field or modify it that specifies dependent custom or default field being not empty, or more specifically, if it has a certain value selected.