Keep the structure visible as you expand. After your first set of ideas, remind yourself which category you are still exploring and whether the interviewer is asking for more depth or a genuinely different angle.

You can also rank ideas as you go. Say that the first two options are likely highest impact because they address the main driver, then add a second tier of ideas that are more experimental or longer term. That makes the brainstorm useful rather than merely longer.

If you are running out of ideas, shift perspective instead of producing random variants. Think by customer journey, stakeholder, channel, cost versus revenue, or short term versus long term, whichever fits the case.

When the interviewer stops pushing, summarize the two or three ideas you would test first and why. The goal is breadth plus judgment, not an endless list.