Of all the theories in this thread, I think this theory has the most merit. However, I don't think we will ever have one true world currency. I think it would be more like you said where we could see more "Euro style" experiments taking place throughout the world. Perhaps we will see a "Pacific" currency where China exerts a bit of control over its neighbors. But I don't see a One World Currency (OWC) ever coming to fruition.
1. A OWC implies that everyone is on board. I just don't see this happening. Places like Hong Kong that have a strong currency and economy are not going to want to switch. And the US is never going to want to give up on its own currency. Too many governments see the benefit of being able to run their own printing presses. They are addicted to it.
2. As we've seen with the Euro, having a shared currency really needs centralized oversight of your economy and spending. This is probably the biggest flaw in the Euro system. Greece has fought tooth and nail to keep this from happening. And no country wants someone else telling them what to do. Can you imagine the Muslim countries in the Middle East bowing down to some committee made up mostly of Westerners? It ain't ever gonna happen folks. Heck, the Muslim countries can't even get along with themselves.
3. Which brings us to the point where we realize that a OWC implies quasi World Peace. You can't have a true OWC unless everyone is on relatively peaceful terms.
4. The failure and faults of the Euro system are fresh on everyone's minds. I don't think anyone is looking at the Euro and thinking that it was a great idea. Perhaps it is workable and they could tweak it a bit to make something better. But most people would be wary of it.
I can definitely see the US losing their reserve currency status. And I can definitely see more experiments with regionalized currencies (Pacific Dollar, Mid-East Dollar, African Dollar, etc.). But a true OWC seems a bit of a stretch to me.
All of this is just my opinion and I am probably wrong just like I always am.