I think it's too early to count out Wang Yang. He did make a rather high profile visit to Tibet for its 70th anniversary of peaceful liberation this year. Considering the significance of that occasion and he was leading the delegation, it would be premature to dismiss his chances.
I am not discounting Wang Yang's competence, after all you don't become Party Chief of Guangdong and Chongqing without proving you have substance and character. However if we take a step back, Wang's rise to the Standing Committee can be seen as a result of the purging of Bo Xilai. In other words Wang took over the "void" Bo left behind.
It is also uncommon for any party member to serve more than three terms on the Politburo since Deng Xiaoping reshaped CPC party politics. Although I think Wang stands a good chance at breaking that political norm next year, I doubt factional politics allows him to be promoted to Premier despite his competence and suitability for the position. If he does indeed stay on for a forth term on the Politburo, he probably keeps his number 4 position on the Standing Committee as Chairman of the CPPCC while Li Keqiang remains number 2 but is "demoted" from Premier to Chairman of the NPC taking over for the outgoing Li Zhanshu... just like Li Peng did in 1998 when he was replaced by Zhu Rongji as Premier but kept his number 2 position on the Standing Committee. That is assuming Xi keeps Li Keqiang on the Standing Committee of course.
Most importantly promoting Wang Yang to a higher profile job, like General Secretary or Premier, would hinder party factional politics. Although Wang has shown to have a pleasant working relationship with Xi, he does not come from Xi's faction, but rather the Hu/Wen Communist Youth League faction. Wang's views might be seen as too "liberal" to Central Committee members that are hardcore Marxist-Leninist Wang Huning types as well as those in Xi's own faction. I would speculate Wang Yang and Li Keqiang's place in the current iteration of the Standing Committee were only secured as some sort of handshake agreement between Xi and Party Elders so that the CYL maintains some sort of representation... it just so happens Xi developed a decent working relationship with Wang (at least one that's more healthy than Li Keqiang) over the years, and Wang apparently tries to stay clear of faction politics despite his CYL roots. As such I won't expect him to rise (much) further from his current post. As someone else mentioned, age also isn't on Wang's side unless an exception is made... which I doubt, or else current Vice President Wang Qishan (who quite possibly is Xi's biggest political ally) would still be on the Standing Committee and at the heart of the party's policy making right now.
As for Wang's highly publicised visit to Tibet, well quite frankly the Chairman of the CPPCC manages the party's United Front strategy where non-party members, entities, and organizations provide input and feedback to the party's policies. With Tibet being an Autonomous Region, it falls into his work portfolio as far as the United Front and ethnic affairs is concerned working with the various delegations that represent Tibet's interests in the CPPCC. Wang also sits on the party's Finance and Economic Affairs Commission that crafts policies for poverty alleviation and the promotion of Common Prosperity... Tibet isn't exactly the most prosperous part of China now is it? As such it is well within party protocol for Wang to accompany Xi on his visit to Tibet. People, especially "China watchers" that really don't know much about how the CPC works or inner-party dynamics, are only making such a big deal out of it because Xi hasn't explicitly named a successor. There really aren't any tea leaves to be read based on Wang's "high profile appearance" accompanying Xi.