It is known that some (non-relativized) syntactic complexity classes between ${\bf P}$ and ${\bf PSPACE}$ have the following property, ${\bf P} \subseteq {\bf CoNP} \subseteq {\bf US} \subseteq {\bf C_=P} \subseteq {\bf PP} \subseteq {\bf PSPACE}$. I am wondering if there exists a (non-relativized) syntactic complexity class ${\bf X}$ such that ${\bf PP} \subseteq {\bf X} \subseteq {\bf PSPACE}$? What are the implications of existence or non-existence of complexity class ${\bf X}$ ?  
Syntactic Complexity Class ${\bf X}$ such that ${\bf PP} \subseteq {\bf X} \subseteq {\bf PSPACE}$
cc.complexity theory;complexity classes
One such class is the counting hierarchy $\mathsf{CH}$. It is defined as the union of a hierarchy that is defined similarly to the polynomial hierarchy:$\mathsf{C}_{0}\mathsf{P} := \mathsf{PP}$,$\mathsf{C}_{i+1}\mathsf{P} := \mathsf{PP}^{\mathsf{C}_{i}\mathsf{P}}$$\mathsf{CH} := \bigcup_i \mathsf{C}_{i}\mathsf{P}$The counting hierarchy has a nice syntactic characterization due to H. Vollmer and K. Wagner Recursion theoretic characterizations of complexity classes of counting functions, Theoretical Computer Science 163:245-258, 1996:$\mathsf{CH}$ ist the set of $0$-$1$-valued functions in the closure of basic arithmetic functions $0,1,+,-,\cdot$ under composition and bounded sums.