There are 9 countries in the European Union outside the Eurozone: Poland, the Czech Republic, Hungary, Romania, Bulgaria, Croatia, Denmark, Sweden and the United Kingdom.
I think that Poland, the Czech Republic, Hungary, Romania, Bulgaria and Croatia will adopt the Euro when they meet the Euro convergence criteria ( Maastricht criteria).
Denmark and Sweden will probably organize a referendum on the adoption of the Euro and I think that the population will vote against adopting the Euro. Therefore, I think that Denmark and Sweden will remain outside the Eurozone.
I think that the United Kingdom will also remain outside the Eurozone. The United Kingdom is not interested in adopting the Euro. I think that the majority of politicians in the UK are against the Euro.