I found this article by @thevladcostea a pretty good survey of the factors at play; I believe he has a degree in International Relations so a nice foundation to reason from.

I'm personally rooting for Iran because I'm half-Persian but they aren't very politically reasonable so I find it pretty unlikely.

I think smaller mostly dollarized countries with benevolent dictatorish leaders (like El Salvador) are most likely to make the shift - they can unilaterally make the decision and they don't have anything to lose. Any country with an existing currency would have to be interested in relinquishing that power which is implausible.