i This isnt ffxi two handed grips dont even exist in xiv yet so i think your getting ahead of yourself there but Rdm not using a shield would makes sense since parry and shield block dont stack currently.
I cant say i want to see a tanking Rdm maybe temporarily in battle since that would fit in with its style of taking emergency action when things go horribly wrong but thats about as far as i'd like to see it go.
