Quick Note:
I only explicitly mention Domains below, for simplicity, but I consider this idea to also include Default Values, since they function essentially the same way.
Problem:
Let's say you have an existing Feature Class that has some number of Subtypes enabled. Now, you go to add a new field that has a coded value Domain, so you specify that Domain on the Fields View, and go to start editing.
Wait, why isn't my domain activating? Oh, right: I have to go add that domain to every single Subtype manually.
If you have one or two Subtypes, this is tedious, but not an issue. If you have a large number of Subtypes, though, this quickly becomes a big ask.
I know I could do this programmatically with Python, but interacting with Subtypes in Python is frankly a bit of a pain, itself, and I pretty much always have to burn R&D time navigating that pile of nested python dictionaries and re-teaching myself how Subtypes are stored in python before I can get around to actually doing anything with them.
Idea:
Much easier would be a button on the Subtype and/or Field View that lets you specify a Field and a Domain and populate that Domain out to that field for all defined Subtypes.