I reckon D4 would be a known money spinner. there has never been a Diablo that hasn't made a shit load.Diablo 3, in the end, cost me nothing. Sold a legendary Skull Grasp for $50 odd which payed for the game. Almost 1000 hours later I enjoyed my time with D3 but as others have said adding one new class isn't going to do anything to the game except add another grind for those still playing.
They need a D4 but it's probably not high on their list of priorities right now.
They can make it like WOW and have the fan base keep paying for updates and stuff. It could be mega lucrative for them IMO.
Another class would be ok but not great I agree.