Well, night elves are perfect race, Tyrande is a night elf, so she got a "common" night elf model. It's that simple! Mooncloth is not just a "lvl 59 robe" but traditional priestly robes. The Moonglaive is the traditional weapon all sentinels (yeah, all night elf guards have them since they are sentinels) and priestesses of the moon are trained in combat with. So I think that there's no need for a "new Tyrande Model".