I also have the need for conditional fields. The suggestion above to use slices does not help us much, because we use the slices on a higher level of abstraction.
One of my specific use cases is a two column layout that can be filled with text and images.
There are some general fields such as a background for the entire columned layout and then there are specific fields for each column. Depending on what type of column it is, e.g. image or text, the user should fill out different fields.
Since the conditional fields are not available yet I solve this by adding more information to the fields, to indicate which fields are relevant for each column type. However, this soon becomes hard to keep track of and I become reluctant to add more fields, even though it would be useful. The user experience would be much better if we could have a conditional field that added some fields. You could draw inspiration from how fragments work in Graphql.
