I back the api idea. If anything, this helps you seperate logic in the app which will probably allow growing a dev team more efficiently. More importantly, though, is if customers actually do extend your app via an API you be empowering creativity outside your vision while still in your control. Letting users change the scope of your site is about as close to mapping your site to their needs as you can reasonably get.

The API idea is excellent, thank you.

I've had 2 ideas already taken by the big guy who can implement stuff quick (but sloppily). I'm not doing anything that earth shattering. Their user base is hard to chip away at when they match me step for step.