I stand to be corrected
as the aforementioned Iceland arena is not in the list from wikipedia.
But these are all European arenas, in cities the band have NEVER played a show. There are of course many more cities that the band have not played for a good while with suitable arenas. I've picked a cut off point of 12,000 capacity. Also I would guess a few of these arenas are actually not on the 'gig circuit'
Add the obvious cities in and who says U2 can't do a proper European arena tour!
Won't happen but it gives you an idea of the scope available.
Kombank Arena 25,000 Belgrade Serbia
Baku Crystal Hall 25,000 Baku Azerbaijan
Ethias Arena 20,000 Hasselt Belgium
Bizkaia Arena 18,640 Barakaldo Spain
Kraków Arena 18,000 Kraków Poland
Žalgiris Arena 17,500 Kaunas Lithuania
PostFinance-Arena 17,131 Bern Switzerland
Fernando Buesa 15,504 Vitoria Spain
Malmö Arena 15,500 Malmö Sweden
Arena Armeec 15,000 Sofia Bulgaria
Minsk-Arena 15,000 Minsk Belarus
Jyske Bank Boxen 15,000 Herning Denmark
Arena Stožice 14,480 Ljubljana Slovenia
Romexpo 14,000 Bucharest Romania
Atlas Arena 13,805 Łódź Poland
Sørmarka Arena 13,000 Stavanger Norway
Elmia Eventcenter 13,000 Jönköping Sweden
Adriatic Arena 13,000 Pesaro Italy
Flanders Expo 13,000 Ghent Belgium
Arēna Rīga 12,500 Riga Latvia
Galaxie d'Amnéville 12,200 Amnéville France
Palais des sports 12,000 Grenoble France
Ice Palace 12,000 StPetersburg Russia
Siemens Arena 12,000 Vilnius Lithuania
Spaladium Arena 12,000 Split Croatia
ČEZ Aréna Ostrava 12,000 Ostrava Czech Republic
Bolshoy Ice Dome 12,000 Sochi Russia