The choice of the escape room theme is one of the most important decisions for escape room owners. Catchy theme can be a motor for advertisement, attracting customers on it's own, while the boring and non-appealing one will turn your escape room project into the uphill battle. Would you rather visit an "Intergalactic Ninja Dungeon" or "Accountant's office"? I bet we all know the answer.
Besides "catchiness", the theme is as good as the amount of marketing it already has under the belt. If you come up with really fully unique setting of your own - you will need to explain the customers why it's cool (which might be tough as well). If you pick one, which is already promoted by others for years or even decades - that makes your life significantly easier at least on one occasion.