My understanding is that Xi wants to be in the office so Taiwan can re-united within his Presidency. This will be considered as the whole reunification of China which carries the ultimate importance in Chinese value system and history. Taiwan itself in that process is _NOT_ relevant, it is an island generating 3% of China's GDP with less than 2% of the population. However, a bold however, being able to do that implies the largely withdraw of the American influence from the west pacific. It is THE symbol of the rise of China.
He is not just asking for his 3rd term. President Xi Jinpin is asking to be the leader in charge to manage and witness the above mentioned transformation.