Personally I would love it if I could have a screen where I could input all of my expenditure for that month. It could be backed up by the direct debits and standing orders known to Monzo, but with the ability to edit the amounts as some do change.
Then I could add things I would be spending on the budgets and should then leave me with an amount I can safely spend.
Also the ability to ‘catagorise’ a transfer to a pot (or at worst a payment to a credit card). I plough most of my spending through American Express and I usually put the amount into a pot so that it’s out of my balance. Being able to just tag that as what I spent it on would give any budgeting tools use for me.