i think yao has the potential to develop into an even better player. at the end of the season he started to slow flashes of physicality. he played much more aggresive down low than you normally would expect of him. if he unleashes the beast inside of him every game he will be unstoppable.
His assist and blocks tend to fluctuate so far in his career, but I'd say in his prime he'll get a max of 2.5 assists and 3 blocks.
His points and rebounds should definitely increase. His season average was 22.3/10.2, but he was tearing shit up after his toe surgery. Look at his last 20 games. He was basically 26/11/2/2. That's considering that he tailed off in his last 5 games, to the tune of 18/9/1.4/1.6.
So essentially he's already shown us that his prime could be at least, if not better than 26/11/2/2, if he has a fully healthy season.
With McGrady back in the lineup, I'd say his scoring doesn't stay quite as high as it was post-surgery, but I think his assists go up a little.
he will peak once he has a very effective inside-outside game and can play multiple defenders from different parts of the floor.. much like Nowitzki, only I don't expect Yao to do anything more, such as dribble penetration, than to play from the high post as well as from the block.