For when paying back large amounts on eg a monthly basis, and to make sure you don’t overpay, an option to have for example:
Pay £50 a month until £500 has been paid
So that you know only £500 will be paid, and not £550 (or more) because you forgot to cancel the standing order.
(Bonus points for the ability to see at a glance how much has been paid so far, how much is left to pay, and the ability to change the amount/frequency paid if wanted)
When setting up a Standing Order you do have the option to set an end date for the payments so you just need to work out how many payments will need to be made in order to pay the total amount.
It probably is more intuitive to set the payment amount, start date & frequency though and then have Monzo figure out when the end date should be in some cases though so I like this idea 
it’s all about the details..
To follow up on this (although I’d love my first suggestion to be implemented) I’d also like to suggest an improved pay until selector.
If you have it repeating every month on the 1st of every month, it would make sense for the end date selector to be
1st October
1st November
1st December
1st January
Etc
And make it clear whether or not a payment will be made on the 1st November if you select 1st November 