How would you make Mime work as a Job? (Even though it probably never could)

[Steady]

Easy: Can utilize skills from all jobs already leveled, limited only by what you can fit on your hotbars. Class skill/weapon skill restriction rules apply (that is; if you can't use it normally as cross-class, i.e. Warrior's Overpower, Mimic can't use it at all).

It would be broken, goofy, and ridiculous, but plenty of fun.

So you picture it more like a Marauder using Crossclass skills for his DPS?

See I would think you'd need to have one Flat stat that affected everything, and have stances of some sort that adjust between one set and another. Perhaps a changing hotbar.

In order to be effective beyond a novelty for any role other than dps (and maybe including) they'd need access to more things than just cross class items. There is just not enough there I don't think.