Imagine for a moment if you will

[linay]

Honestly, you can get rid of class/job levels altogether and just have item levels. That keeps the sense of progression without the need to change/rebalance job each expansion due to increasing job levels (though you can still add or change skills as desired).

EDIT: As someone has pointed out:

In theory ilvl would keep progressing as it is now, our stats would continue to grow with gear.