If there were one area you've always wanted to improve upon, what would that be?

Everyone has some strengths and some weaknesses. The interviewer wants to know one of your weaknesses. This does not have to be something big but refrain from mentioning something personal. You might be real bad at dating but this is not what you want to mention here.
Go through the job description and see what are the skills that are needed for getting the job done successfully. Find some skill that might be of very less importance for this profile and feel free to mention that here. Also, mention what you are doing to improve yourself in that area. Most importantly, stay positive!