You can improve the Tails user interface and user experience.
The tasks marked as user interface design in Redmine correspond to things that we would like to see improved in the Tails user interface.
If you are a UI or HCI person, you can surely find other ideas to improve Tails! Even if you don't implement your suggestions yourself, create tickets with your ideas so that others can benefit from your insight.
You can subscribe to the tails-ux mailing list.