No high-income country has shown serious interest in joining the EU in decades. Norway is a hyper-privileged nation that already has several relevant treaties with the EU and has very little to gain by submitting to a system where the Balkan gets to vote on the measures Norway must implement.
Is the top not an article about a high-income country expressing serious interest in joining the EU?
are there any more high-income countries in the European geographic area that haven't already joined?
Nobody wants to risk when things are going well, obviously. These countries do, however, have a huge number of bilateral agreements, so they find a lot of benefits in joining, at least partially, the EU. Norway already implement most of EU regulation without having any voting powers.