@esmi - thanks for the reply. This wouldn't amend any of a theme's original files at all, it would create new ones.
Let me explain the problem.
Scenario: you have a WP site with 10 categories and 50 tags. You would like to take advantage of those pages by adding original content and possibly monetize particular ones with specific ads by category or tag. By creating a cat-id.php or tag-slug.php file (based on your index.php or single.php) and placing it in your theme dir, that file will be get served as the theme page for that particular tag or category page.
I guess that you "could" store the specific content for a cat or tag page in the db, but I didn't want an "additional" db query. I mean a WP page is already getting "the_content" for the post/page anyway, and the plugin would have to grab the specific cat/tag content and post it before the regular content (double lookup).
But I do now see what you mean about "switching themes" - the tag/cat pages you created would have to be created again for a new theme.
Any ideas? AFAIK there is no plugin out there that help to manage tag or cat pages at all.