~$280T Global wealth 21M Total Bitcoins
Assuming Bitcoin stores 10% of the total value of domestic companies, this gives
(0.1) * ($280 * 10^12) / (21 * 10^6 BTC) = ~$1.3 * 10^6 / BTC
$1.3M per Bitcoin.
This gives a rough range of what the Bitcoin value should be. Increasing the stored wealth increases the number as would an increased global company market capitalization. The 10% was chosen as an arbitrary number. One can easily imagine 20%+ or more as also being reasonable.
As a guide, if Bitcoin should ever significantly exceed the global wealth or still be volatile after being valued at a significant percentage of the global wealth, then this would indicate, to me, a bubble.
As has been said elsewhere, should Bitcoin store a significant amount of wealth to warrant a $1M+/BTC, this would probably come with some significant changes, such as:
The point being that Bitcoin adoption doesn't happen in a vacuum. If the price were to rise significantly, this would bring changes to our everyday lives including the way we do banking.