HF, Aditya has got it right. What used to happen is that users would make (big!) changes to DocTypes and when we made changes, they used to conflict. Also sometimes changing the DocTypes would cause errors in the code etc.
But you are right, there should be no reason not to allow now new doctypes ---- we will devise a way to separate them from non-standard DocTypes and so the users can manage on their own..
On third thoughts, if some one wants to really make a new DocType (and know some code), they can create their own "fork" on GitHub! Maybe some of those new DocTypes could find their way into the product ;)
Also what kind of changes are you looking at? If you have specific examples, maybe we could teach you how to put it in the product too
best,
Rushabh