I did a little poll some time back that might give you some ideas. Unfortunately it was consigned to oblivion in one of the mega threads Monzo merged any posts about categories into. You may find some ideas there though. I agree this would be a nice idea, and many many customers have asked for changes over the years. They did add two this year I think.
The reasons given for not expanding were that it makes the design (for choosing, for displaying spending) more complex, and it’d make classifying data across customers more difficult. Personally I’m unhappy with the current spending design as it doesn’t display all the categories I’ve used in a given month, only a few, and then displays lots I never use (why?), I’d go for more sub-categories if they don’t want to expand the top level ones and display all the ones used in a given month. Tags are not a good replacement for categories because they don’t have auto-classifying - I have no interest in manually classifying transactions, maybe others are ok with that, if so I think tags are probably ok for them.
Savings is the big area I feel like they’re missing a trick on - at present saving is treated as spending, which is just wrong in so many ways, particularly when they want to encourage financial responsibility. Might take a while and a few more savings accounts in marketplace before they tackle that though. Anyway, lots of ideas here from customers, but little movement from Monzo on it unfortunately.