Welcome.
Before you start contributing to App Inventor Open Source, I suggest you to understand about App Inventor, how it works, and its structure. @RamNaresh gave you a list of potential projects that you can use. Before you start, play around with App Inventor Sources in GitHub, for example, components.
I have never joined GSoC, but I think if you want to contribute to App Inventor via GSoC, you can take a look at previous proposals and learn from them. Also, look at other builders and try to find out what extra features they have, for example, custom font support (already proposed), helper blocks (already proposed), title bar menu items, etc. Think about useful features that can make App Inventor even better. You can also use the Site Feedback category to find some user suggestions to App Inventor. Or, read guides in the Open Source Development category about how to make your own App Inventor.
Take a look at these proposals.
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What is *your* opinion about helper blocks in App Inventor? (GSoC Project - User Feedback)
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Request for User Feedback on the Animation Improvements Project
This gives you a general idea about the languages App Inventor uses. Taken from Peter.
Have a nice summer.
Gordon