Belt drive would not work with the present swingarm and would require a total redesign. Shaft drive is nice but it adds weight.
Well, they did a complete redesign of the Ninja 650 so it's not out of the question. On the other hand if a factory centerstand were available I would have no issues with the current chain drive.
There are other issues with a shaft drive but I had one on my 84 CB650 and for me the positives outweighed the negatives.