Visa and Mastercard are working on a card tokenization, and I wondered if it was possible with this token stored on a NFC-tag like a key-fob to perform a contactless payment?
Do you think it is enough to put the token on the NFC-tag or is it must be wrapped ?
It will work like the visa ring.
Yes, but with a lot of conditions (nothing is simple, sorry).
The keyfob must have a real Secure Element inside that can run an applet that complies to Visa/Mastercard specifications. A simple NDEF tag would not do: this is not just about storing some data, but it is about an application handling it securely.
In other words: the 'token' has to be stored securely on the chip, and the chip shall send it only to a reader that has authenticated itself properly to the chip. The specifications are freely available, have a look at 'EMV'.