With nearly £1bn of unwanted foreign currency ‘wasted’ at airports, perhaps Monzo could look to provide a solution to the age old problem of getting rid of your foreign exchange before heading home
£1bn figure reference: http://home.bt.com/lifestyle/money/money-tips/nearly-1-billion-of-unwanted-foreign-currency-wasted-at-airports-11364160531017
I’m not sure how a digital bank could solve this problem to be honest. Unless they let you deposit it and then convert it back, but I don’t know if converting money costs Monzo money.
Monzo do solve this problem, no? If you use Monzo instead of foreign currency then there’s zero wastage.
Well, you use Monzo to take out a bit of cash in ATM for an emergency… And because Monzo is great and you never use your emergency cash, you are now left with it at the airport. So, basically, Monzo is amplifying the problem by being so great. 
But this aside, Monzo will limit the amount of random currencies, but I think I already have EUR, PLN, CHF, RON and some MAD I forgot in my pocket and smuggled out of Morocco illegally. All sitting in my drawer. I only have CHF because my flight was cancelled and I got a different connection via Switzerland. Since I don’t know when it would happen again, I’m probably going to keep CHF forever in my drawer.
It does bother me a bit. I’d be happy if Monzo could take it, exchange sensibly (doesn’t need to be best market rate) and put GBP back into my account. I do feel however that could be expecting too much from Monzo as a bank to take care of that. 
edit: I’m gonna try this one out: https://www.leftovercurrency.com/