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.
This will depend on the development of bitcoin. If bitcoin remains highly volatile and cannot escape the speculative asset label, it remains subject to manipulation and is frequently dumped whenever there is instability. I don't think banks will add it to their reserves. Because what banks care about more is stability and safety, and that is why they still choose and prioritize gold over any other asset.
Also, this could happen but not anytime soon as central banks are still skeptical and wary of bitcoin. The Fed, ECB, PBOC or BOJ, the world's leading central banks have yet to express interest in bitcoin, let alone add it to their reserves in the near future.