Banks use tools as a store of value by holding some assets in reserves, for various economic benefits. Although, adopting Bitcoin in such capacity, would have it's pros and cons. Obviously like controlling inflationary trends, and even help stabilize the volatility of Bitcoin to its users, and conversely such volatility could also alter the stability of a banks statement of financial position. All these could add up to a more stable global economy. Surely banks have started to think of such prospect
Someday it will happen that most of the banks in the world have a reserve with Bitcoin. It won't just be gold of what they have in their reserve.
That is a big possibility that it's only a matter of time of when it will happen. Because the attention that's given to Bitcoin is already high and quite well enough to be recognized to be used in diversification of these establishments.
While there is no forcing that's happening for them, it's where the big money is flowing as well so they won't ignore that fact.