Personally, I would love to see the feature to request money from other mondo users. When someone owes you money, it can sometimes be an awkward conversation to have. If you were able to use push notifications to prompt someone, it would be useful.
You could use light humoured things setting an atbitrary APR to make them pay up. If the owed money even started gathering interest this would make them pay pretty rapid!
Keen to know your thoughts.