I know the idea is to make all payments digital, but there are still times when I need a bit of cash on me (buying from market stalls etc). So I still take a little cash out from time to time.
One of the strong points of Monzo is the categorisation of expenditure, for budgeting purposes and to keep track of spending. With cash however, it just disappears without you knowing how you spent it.
It would be great if you could split money taken from ATM withdrawals, assigning portions of that cash to specific categories. So I take out £20 via ATM, that gets added to an unassigned pot, then I buy a burrito with cash at the local food market for £6. I can go to that pot and assign £6 of that unassigned money to an eating out category, perhaps even, searching for an establishment, adding a map etc.
Personally I think this would be useful, especially for those where budgeting and visibility of spending is very important.