A new country must become a full member of the alliance for this to count. Which country that is is irrelevant, even if it is a new country that has seceded from a current member.
Potential candidates…
Bosnia - unlikely due to SR/Serbia entanglement risk
Ireland - low popular support to end neutrality, could change if there was some kind of Incident with Russia
Austria/Switzerland - probably same as Ireland but with even greater costs to dropping neutrality
Cyprus/Malta - would need some kind of geopol catalyst in the mediterranean
Ukraine/Moldova - No due to Russia war risk
Georgia/Azerbaijan - No due to lack of US appetite
Mexico - weirdly seems like one of the more plausible routes if it suited domestic player’s agenda
misc Asia/Pacific members - would take a few more years to cook up a strategic shift like this
any I’m missing?