I have some random ideas:
1. Allow multi-answer for multiple choice questions. There is a flag to indicate whether a question is multi-answer and this flag is set by the question author. Players will see a message like "please select all correct answers" for this kind of questions -- even the question may have only one correct answer actually.
2. The system may record the number of trial and correct attempt of each questions such that there is some ways to reflect the difficulty of the questions. This piece of information may be used to evaluate the scores where more difficult questions have higher weight.
3. The system may have many questions in a quiz and each time a fixed number of questions randomly drawn from the database to the quiz taker.
4. The system allow plug-in for different quiz types, e.g. Millionaire quiz, timed quiz, etc.
Hope that I am not too demanding