For an example of this in an existing app, see Ulysses, where the first Markdown header or the first few words are used to generate the note titles. This would obviously be a bit more complicated in Joplin’s case, since it would have to overlap with manually set note titles in some sane way. My thought for a simple first version would be to have it as a global preference that just updates the note title whenever an initial Markdown header or the first paragraph of the note is edited.
Unless I have misunderstood you, doesn’t Joplin do this already?
I have found that if you create a new note (Windows, Desktop, Version 1.0.175) and do not add a title first, the text of the first paragraph is used to create a title. It seems that if you change the text of that first paragraph it will also change in the title field until you move off the note whereupon it becomes fixed.