I think that it's a huge mistake to try to describe everything with mathematical formulas, this is just one of the components and it's not the most important one.
The problem is that price is a number. Thus, if you want to get to a theory about how the price or value mechanism works, a mathematical formula has to be employed.I consider MV/PQ to be an approach that's worth a try. However, a "Grand Theory of Cryptocurrency Value", as you wrote, should include more elements. In my opinion, above all, the size of the ecosystem of groups and organizations/businesses related to the particular cryptocurrency should be estimated to get an impression if the user base is growing or not, and at what (approximate) speed - transaction data alone is not enough.
Another interesting approach would be to study the "hodl/transaction" ratio in relation to price movements. For example, in bull markets, "hodlers" numbers - as far as I know - tend to increase, while in bear markets and phases where the price stagnates, the usage as a transactional currency grows. That, however, has to be studied thoroughly. If someone knows empirical research of that kind, feel free to post it here.
