This question is obviously asking you to look forward to the future, so drawing up one year, five year and ten year research plans is helpful before going into an interview. (If you are completing a PhD and looking for your first job this might sound a little excessive, but believe me, it will make you stand out and be taken seriously). They will not just want to hear about the topics you wish to cover but will crucially be interested in two other areas: research funding and your output (i.e. publications). As well as telling them about your exciting projects, tell them how you're going to pay for them (which grants you will apply for) and what published benefits there will be in concrete terms. Aim high at this point; you don't want to sell yourself short. You can write that monograph or get an article into a world-renowned journal. The scope of your ambition will help to sell you as the ideal candidate here, so don't worry about sounding arrogant.