Its not about the perfect answer. There are many many good answers.

When you hear "what-else", there could be two things:

• Interviewer is not convinced that you have covered all areas- so refer to your structure, take some extra seconds and fill the gaps
• Interviewer is stress testing you- if your structure is comprehensive, stand your ground and defend it. Otherwise, do as above 

Ideas/Solutions need to be realistic and executable. Not everything can be implemented and not everything can be implemented on Day 1. So you need to go a step further and apply some prioritisation rules to provide a realistic plan for the ideas/solution.

The "what else" is not always necessarily because the interviewer is pushing for a specific answer. This is also the wrong way to approach a case interviewer. It's not about coming up with THE answer but more to share how you think about different potential "answers". As a result, breadth and depth of thinking are equally important.

It's also worth noting that the "what else" is often a convenient default for an interviewer to fill time and/or just see how you react under a little more pressure.

(editiert)

Hi there,

I highly recommend that, when asked for "what else" you don't just keep listing things, but also refer back to (or create) a structure. They're trying to see if the you can tackle the problem logically+with structure, not just if you can come up with a million ideas!