might I suggest...
 

Might I suggest that a new template item named with a reserved word "default template." No other template could have that name.

Now when I open a new tab, it opens a new instance of the default template.

Make it so I could have existing or new item as the default template, but once I have a default template in the Templates section, we're done.

Alternatively, and not being a programmer, I don't know which is easier to do, make a template attribute called "default." Make it so that only one of the templates can be designated default. Now when I open a new tab, let it open an instantiated item based on the default template.