Just an idea for travellers that might be useful for the app.
Providing said traveler has location and data services activated on their mobile device, could the app use your geographical location to provide local knowledge about lesser known banks/ATMs that are known to have charges? Bit like a live travel tip once you hit that location.
How would you want those bank’s /
ATM’s locations to be displayed? In an ATM finder map or some other way?
It’s good timing for your post because this feature will may become more valuable, now that Monzo’s considering applying fair useage fees for ATM withdrawals.
A map is certainly a slick way to do it 
but even if you just provide a list of banks relevant to the users location so they know which ones are more favourable. Then it would certainly help if you end up somewhere a little less familiar.
I think this is a great idea, especially as in some countries, the ATM user fees (as opposed to those Monzo pays) can vary widely, and even include free options, but for a visitor it’s difficult/impossible to know when/where they are available. This would be an excellent use of crowdsourcing, and could help offset the overseas ATM fees Monzo will soon have to start passing on to us.