Add ability to define ads by category group
Creating generic positions for ads is great but having the ability to assign a category or two would be very useful to allow category specific ads.
I'm thinking the ability to assign category groups to the Ad Groups settings, then when you add a new ad you can assign the categories it will display under.
It might also be able to define a fallback ad if no ad is available for a category.
I would go more farther: Not only one or two categories - but select a variable number of categories. That would be useful if you have a lot of categories and subcategories and want to place ads from different advertisers.
The fallback-option is a very good idea: So you could say "if there is no ad just display my adsense stuff" or anything.