Everything is possible unless it directly contradicts physics.Is it possible in the future for the J-20 to have F-22 like square nozzles?
You can make them square, triangular or whatever you want and can.
Question is wherever it has a point: to make square nozzle feasible, you design plane with it in mind, from the very basic geometry on, and the other way around.
Otherwise, thrust/efficiency loss is going to be very substantial.