Could support for the Wii U Gamecube controller adapter be added in the future?
The Gamecube controller has similar if not the same notch placement on the main stick as the n64 controller. Has built-in Rumble motors, and has enough buttons for everything.
Also the Controller adapter has four ports which is good for multiplayer games.
How this could be achieved I believe is by using some source code from Dolphin which has native Wii U controller support and all that users might need to download besides Project64 is Zadig in order to patch a driver.
Plus the Gamecube Controller is like the Best thing ever.