Just pick an app that you think needs a better UI and re-think the navigation and layout, then use a mock-up tool (e.g. http://www.balsamiq.com/) to create a mock-up and create a new thread and discuss your ideas with the developers.

The developers will then tell you about some special cases and features that you have not thought about in your mock-up. Fix your mock-up or convince the developer that his/her idea of this feature is not optimal.

There are lots of small paper-cuts that can easily be fixed. For example, the fabulous "Desktop Command Execution Widget" has a preferences dialog. In this, there is a picker button "Update interval:" with different choices (5 Minutes, 30 Minutes, etc..). The first choice is "0" and the label below the button says "'0' disables periodic updating". That's just plain wrong - just change the label of the choice from "0" to "Disabled", and get rid of the label. It's even small things like this that you should find and tell the developers about.

Edit: Screenshot: