Best to pick a business you understand and is simple for everyone else to understand… usually a retail chain is easier than an insurance company.
Focussing on a company that could clearly improve is a good thing. When I did my interview, I used M&S and at the time they were in dire need of help. It served me well.
You want to go for just the bigger picture but be specific, e.g. start selling online, hire designers to improve items, redecorate stores, new marketing campaign, targetting new markets that sort of thing. You don’t need any numbers.
Really they are looking for:
Your ability to present well
An understanding of what makes a business ‘good’ vs ‘bad’
[*]Ability to back up and argue persuasively your points when questioned (which you will be)
Hope this helps!