What if when you were synced to content from a particular expansion, you played with the kit you would have had when that expansion was current? 1-50, you get the ARR version of your job. 51-60 you get the HW version of your job. Etc.

I can already see all sorts of problems with this (cross-class skills, juggling hotbars, having to remember five different versions of the job), but it would guarentee having a complete-ish kit in content designed with that kit in mind.