You can't include external data without oracle or someone who you must trust.
Maybe we can use Mining difficulty as a predicting tool for the price?
When bitcoin prices rise, mining profitability will rise, resulting in more people mining, and therefore increasing difficulty. we need to factor in mining hardware improvement though (Updated Moore's Law?)
Interesting idea. but i think it would be unreliable/inaccurate, because
1. Mining profitability also depends on operational cost
2. How accurate we could estimate mining hardware growth? Additionally it'll be less accurate if we estimate far distant into the future
3. Another external factor which affect mining activity (such as regulation and natural disaster)