So -- pure curiosity... Which countries could yet still potentially join NATO.

Switzerland doesn't join anything ever, so it's the dark horse. But since everything is done by referendum there, it could change on a dime if the public demanded it.

Austria literally has it in their constitution that they aren't allowed -- but in theory they could change their constitution (unlikely).

Moldova has the whole Transnistria incentive -- but NATO would be shy about that one, because that could potentially immediately put them in hot conflict. However, suppose they backdoored their way in by creating a union with Romania (not impossible, but complicated).

Ireland has been neutral forever -- but the public support for Ukraine is extremely high. So they might even be possible. Higher than Switzerland anyway ;)

Bosnia and Herzegovina is sort of a special case where they're sort of partially engaged already.

Serbia is extremely unlikely while they continue to be extremely contemptuous of everyone. That's fine. Although Kosovo is sort of under NATO protection.

In theory, Georgia or Armenia would be candidates, but Turkey would pooh-pooh Armenia right away, and Georgia has contested territory.

In order of odds, I wager: Ireland, Moldova (via Romania), Georgia+Ukraine (in that order chronologically).