Within your terms and conditions, you need to define what can and cannot be promoted through PPC. The most obvious restrictions are brand terms and misspellings. So one of your objectives could be to increase your visibility on search engine affiliates, and you can do this by targeting only long tail keywords. Other things to consider are whether affiliates can link PPC directly to your site and whether they can use your URL in the ad copy. These are decisions you need to state in your terms and conditions.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.