I would really like to see some integration with PayPal that allows you to see full details about PayPal transactions and the final vendor for the payment rather than just seeing PayPal in the Monzo app. Would make Monzo more of a hub and also allow to split PayPal transactions up better into the correct categories.
Third party investment support would also be good like trading 212 along with the ability to share a read only view of these accounts with a partner/joint account holder.