Image block already works as such. Adding an image block in a template without adding any images will work in the same way you’ve described.
This would be super useful. I prefer a placeholder text as well. So when the template is opened, it would display the placeholder text but when an object is created from it, it should be empty / Untitled
Templates are currently blueprints for new objects of a type to be created. As such, they are only used when a new object is created. This discussion has links to several different feature requests including template sync which would make templates work in the way you envisioned