Today when I use my Amex at certain retailers (British Airways for example), additional albeit limited information gets added to the transaction such as ticket reference number, origin/destination etc. Similarly if I buy something in the Apple Store I get an emailed receipt containing additional information and with Uber I get details of the trip made. Whilst this helps with warranties, insurance claims, work expense claims and travel planning etc, the additional information is not structured so not readily analyzable.
Perhaps Monzo could offer retailers a secure API through which they could optionally submit additional information about a transaction in a structured way? Imagine doing a grocery shop and getting a structured receipt itemizing everything you bought from which price comparisons could be driven or nutritional information received, allergy or product recall warnings given, or warranties activated.
Retailers could be motivated to share this detail if in return - provided you authorized it - they received your contact information for marketing purposes and perhaps greater insight into what you like as a consumer; whatever you felt comfortable sharing.
You could start by targeting manufacturers of the most common POS systems first.