Don't know if there are any football fans here but i will make this analogy Diablo is like Arsenal in football. With selfsustaining model in which people pay the highest ticket prices for a mediocre performance.
The story here is the same. As the current Arsenal team, Diablo 3 is riding on the previous glory days of old Arsenal team/Diablo 2.
So for those reasons i dont think adding new classes will change anything except maybe adding few more people on the side lines who happened to play the classes that are not currently in the meta.
Diablo 4 might be the answer, maybe they will learn from their mistakes and make a great game once again 2x2=4