Is there any measure that is a non-trivial combination of VWAP and TWAP? For example:
\begin{equation} \textrm{VTWAP} = \frac{\textrm{VWAP}+\textrm{TWAP}}{2} \end{equation}
I'm thinking about something like this:
\begin{equation} \textrm{VTWAP}_{\textrm{exp}}(\alpha,T) = \frac{\sum{P_i * V_i * e^{-i*\alpha}}}{\sum{V_i * e^{-i*\alpha}}} \end{equation}
where $P_i$ is the price at time $T-i+1$ and $V_i$ is the volume at time $T-i+1$.
Influence of past volumes is exponentially decayed with factor $\alpha$.
We can see that $\textrm{VTWAP}_{\textrm{exp}}(0,T)=\textrm{VWAP}(T)$.
I think that good point to start to analyse this problem is to find out types of existing TWAPs.
Second part of the question:
Are there any mathematical requirements or equations that measures like TWAP and VWAP should meet?
Something like that, but more advanced: $\textrm{VWAP}(T+1)=\textrm{VWAP}(T)$ for $V_T=0$ which state that there was no trade at time $T$.
