This is an interesting question.
Perhaps because using the word 'if' presumes that it is not certain that such an question will be asked at all.
Are you therefore anticipating the typical interview questiosn about career aspirations being asked? Or just covering all bases as part of your interview preparation?
If I were the interviewer asking such a question, it would be to dig a little deeper into your thought processing... that is, how you reply, will tell me more about YOU the interviewee, than the actual information or content given.
In other words, I personally would not be placing any real emphasis, or importance on your reply; in linking it with you progressing positively to the next stage in the interviewing process.
Therefore, my suggestion would be that the reply should both surprise, and inspire the interviewer with whatever it is you choose to say.
This I believe is the secret mission in the question.
Aspiration is the child of inspiration - so what inspires you, could and should consolidate into an aspiration.
For example, if you are inspired by powerful orators, then your aspiration might be one day, to affect positively the minds of the many through the power of speech and communication.
A worthy aspiration don't you think?
Here's another example using the same model:
You may be impressed by how the organisation that you are seeking to join, empowers its people, promoting a stake-holder culture where everyone can benefit from pulling as one in line with the business mission.
Your aspiration might be, to one day be viewed as fundamental in using the existing successful model of the company you wish to join, in finding imaginative and constructive ways to create an even stronger culture of success; not just for those working for the enterprise, but also to those to which it serves - its customers.
If you use the above example to think about other aspirations in a career setting, then you will not be far off from making a telling and memorable impression on your interviewer when they ask you typical interview questions about career aspirations.
Good luck, and let me/us know how you get on!