It is not uncommon that for an atypical case, none of the "standard" structures will fit. I once had a case which was more of a detective story. There was an airline that was suddenly having long, long delays at check-in counter and I had to figure out what was going on. No actual structure could possibly cover all options, so I built a high-level basic structure and then once I exhausted it and the interviewer shook his head saying "no, those aren't the reasons for the delay", I had to go back and think more and add other possible scnenarios. So, here's what I think could work generically:
1. Create a high-level structure and if you aren't sure you've covered all possible areas of exploration, just let her know based on more data, you may modify the structure if that's ok with her. Note that this is more likely for an atypical case. I don't think you'll end up doing that for a profitability or an M&A case e.g..
2. Go through the top-level analysis quickly - exhaust the branches of the tree. The "redflags" to look for here to know if you've missed something big are: 1) there is no data available for your branches and you can't go a level deeper anywhere 2) the interviewer's body language / any verbal feedback.
3. If #2 isn't going well, stay calm. Say something like, "it looks like we may need to analyse other focus areas - let me take a moment to modify my earlier structure". Then come back and repeat 1-2.