This is a question which comes up in all job interviews. Your employer is checking your loyalty and commitment to the firm.
Give an answer which indicates a slow and steady career progression within the company.
An ideal in answer for a technical project manager could be as follows:
I hope to still be a project manager but more senior, and with more experience working on a more diverse range of IT projects. My long-distance plan is to ultimately become an expert in Agile Development and transition into a Scrum master and give mentorship to my scrum team and the other project managers.