Solar sails offer the opportunity to travel between planets with no consumables, extending mission life and possibly opening new destinations. However, the technology is still nascent. A challenge that would unlock new avenues of exploration while being achievable in the next decade:
Using only a Solar sail for propulsion, navigate a spacecraft from low Earth orbit to low lunar orbit.
This challenge would require advances in material science, navigation, and spacecraft autonomy. If successful, these technologies could dramatically reduce costs to explore our Solar system, opening up other destinations the way cubesats have opened up LEO-related investigations.