Include an editor so that users can create their own templates rather than having to purchase new ones from you - that would make the program much more attractive to the general user.

Isn't it so that the "general user" of this kind of programmes just want to make quickly some business cards or flyers and needs templates? Not everybody is a graphic designer; the designers wouldn't use such a programme, they make their templates from scratch with more sophisticated DTP tools - more sophisticated and (mostly) much more expensive. It's no good idea to blow this simple tool up with an editor.