@Nick The N26 Debit Mastercard is a real debit card and not some hybrid card. It is processed as debit in the payment process and it will also list as a debit payment on your receipt. Mastercard/Visa or online use is not limited strictly to credit cards. Debit cards can also be MC/Visa or used online, even Maestro (although not all banks support the online use of Maestro).
A debit Mastercard would make bunq much more attractive. Especially in the Netherlands where no banks offer those cards. And since it does not involve credit it would not breach the 'no-credit' principle of bunq.