What other factors apart from supply are quantitatively known?
Well, that's why I made the thread to ask this question. I think a lot is known. For example, in my opinion, a small part of the reason the value is rising over time is due to Bitcoin's scarcity and alternative currencies supply expansion. So, not only is supply of Bitcoin known, but perhaps there's a proxy for M1 expansion too (if we're talking dollars).
Another variable that's known is "volume", for example.
I wouldn't say that its easier than Forex. FX long term forecasts has multitudes of macro economic data going in. For the short term, the price history depth makes autoregressive models possible. Neither of these factors are available in BTC.
I guess we disagree a bit here. I would say it's easier than Forex because it's not tied to an economy. While you believe being tied to a country simplifies things (because of data - don't get me wrong, I see your point), I use to model economies, and I believe being tied to a country can complicate things. Although more data may be available, the "right" data is not and it's not very reliable. Supply and velocity are largely unknown with domestic currencies because of cash. And the demand issue remains. Bitcoin solves the supply problem because it's quantifiable. In fact, I've created weighted exchange rate models in the past and I think this is quite similar. There's no reason to think that the variables for a simple OLS regression aren't available, which could be used to predict price. I know for sure it's tried in Forex markets. Anyway, if the results end up significant with a nice R-square and coefficients following logic, it's applicable.
And from there, that model could be used to predict alt coin prices in a top down simultaneous model.
So, back to what "other" variables we could use...