Hello,
The idea would be to have fieldtypes with dependencies. For example, if a certain Boolean is set to true, new fieldtypes will become available. A perfect example would be an Entity with the boolean "has expiry date" when set to true, a required date/time field appears.
I am fully aware of potential work arounds with a listner component and a fieldsets "certificate with expiry date" "certificate without expiry date" and so on... but I really would think this would a neat feature to have making life more easy (also less "custom" development).
Could "easily" be accomplished/housed within fieldtype settings and some regex values.