Well two old challenges and one new one. The old one is to find the puzzle with the least clues such that each clue is redundant. ie you can remove each clue and still have a valid puzzle.

The new challenge is to find a valid puzzle such that you can change every clue and still have a valid puzzle. ie if the puzzle had 25 clues then there are 25 valid puzzles which differ from the original puzzle by one clue and each one differs by a different clue. This might not be possible. Then the challenge is to find a puzzle for which you can alter the most clues and still have a valid puzzle.

Also I think we had a similar challenge before. To find a puzzle for which one clue can take on the most values. I think there was one that could take on six values.