I think I have seen a custom design feature request in the forums. Raiffeisen bank used to offer this option (where they operate). Business users with a fleet of bank cards could brand their cards to their likings to ensure the security and safety of their employees. Individuals could use the feature to express themselves on top of their bank choice. Manifacturing all diffwrent cards is expensive, which warrants a premium price of course.
As for the bunq rainbow, I agree, no need to change. However it is widely thought of as support for the LGBT community, no need to travel to India, Iran, or Russia. On my recent trip to Hungary and Croatia my card drew some unwanted attention, which could have easily provoked a confrontation. I felt the need to handle my card with its black side towards curious eyes.
