Which state enters the Eurozone next?

[Joe Republic]

Does Iceland even fulfill the entrance criteria?

Did Kosovo and Montenegro?

When did Kosovo and Montenegro enter the Eurozone?

2002, when the Deutsche Mark was replaced.

[Joe Republic]

Like the others said above, Kosovo and Montenegro are not members of the Eurozone.

Not formally, no, but they are in effect.

[Joe Republic]

No they're not.  Just because they use the Euro doesn't mean that they are members of the Eurozone.

They have adopted the Euro as their sole currency, which means that in an informal capacity they are part of the Eurozone; in that they are subject to the decisions of the ECB monetary policy but have no influence in it, being non-members of the EU.  It's unilateral of course, but it seems to be a comfortable arrangement.

El Salvador uses the U.S. dollar but they're not a member of the United States.

They don't have to be, of course.  Any country can use it if they want to.

[Joe Republic]

I understood your point.  I have already iterated that Kosovo and Montenegro are not formal members of the Eurozone, but given that they were granted permission and encouraged to use the DM before the Euro replaced it, they were kind of swept into it regardless of how the ECB may feel.

The El Salvador comparison is a valid one, but not very relevant.

Anyway, this discussion is a bit of a pointless tangent.