For my own reference, elements that were introduced in Harry Potter and the Chamber of Secrets that I don't think have yet been used to their full potential in the series:

- House Elves (have had their uses, but don't feel fully used)
- Parseltongue (useful in the book itself, but not in the series)
- Gilderoy Lockhart
- Acromantulas (although I hope I hope I hope I'm wrong about this!)
- The Polyjuice incident, and the fact that Hermione got turned into a cat (Polyjuice itself has been used, but could have been introduced another way)
- The concept of regrowing bones
- The fact that the Sorting Hat can be used for purposes other than Sorting
- The Sword of Gryffindor
- The fact that Voldemort resembles Harry
- Colin Creevey and his camera (although the camera did have a function in that book)
- Penelope Clearwater
- The fact that Filch is a Squib