With Estonia into the euro, who would be next? Here's my take:
1 and 2. Latvia and Lithuania - both need only a humming economy to meet all the criteria. They will benefit from the growth spurt I expect in Estonia, so they are best positioned.
3 and 4. Bulgaria and Denmark - both have pegged their currency to the euro, putting them in a position where they have to accept the decisions taken in Frankfurt, while they have no voice in the discussion. The Bulgarian economy is weak, though and Denmark is pretty virulently eurosceptic.
5 to 9. Czechia, Romania, Hungary, Poland, Sweden - a coalition of the unwilling?
10. The UK, if the pound or the economy collapses. Neither likely, nor excluded.
Next question is "when"? I think there will be reforms when the currency dealers finally realise the euro is safer than USD or GBP and calmer times prevail. I think the gnomes of Frankfurt will resist any new members before the reforms are concluded, but will have no problems with Latvia or Lithuania afterwards as long as their stats look good.