This has probably been mentioned but it would be amazing to create your own sub categories for the preset ones. For example in holidays to have the option to group spending by specific location. So you know how much you spent in each place. In transport would be good to see how much one spends on Uber and TFL separately. In shopping grouping clothes shopping away from present shopping or things for th house etc. Anyway you get the idea.
PS. Loving Monzo (can I have a job)