Enable Wi-Fi tethering via Bluetooth?
Performing actions whenever a certain Blutooth device is in reach and paired (and reverting them when it gets out of reach) can be done e.g. with tasker (see the tasker tag-wiki for details on the app):


Condition: BT Connected (parameter: name or address of the device)
Task: WiFi (parameter: On)
Exit-Task: (optional; Tasker should revert automatically to the previous state when the condition is no longer met) WiFi (parameter: Off)


This would similarly work with NFC when using the NFC plugin. More features are also offered by other plugins. Also see my Collection of Tasker Ressources for tutorials and available plugins.