The good thing is that you still have several options:
Since title and subtitle seem to be identical in this case, skip the subtitle.
Use a custom text field with a modified default property as shown above and set the default in the blueprint using query language (the same for any other field types that require query language for the default value)
Use a page model to prefill values at page creation.
I assume the reason why this is only supported for title and slug is because those are the only fields we couldn’t get rid of before in the dialog, while for all other fields we already had the option to prefill them via a model.
Subtitle is used for the meta description in this case. Title is just prefilled, i usually change that before publishing. So, i do need two different fields.
Tried to achieve what you describe in 2 before but it didn’t work.