Although the idea's you are coming up with really do seem plausible, I still believe the only route to take on this matter of Job/Rank Uniqueness would be to keep it within the confines of the job. This theory still has the same flaws of making a job unique. Granted, I'm in favor of making some abilities obtainable through cross matching.

I believe the only way we'll ever reach a level of uniqueness is if most of the abilities of a certain rank stays within that rank. I'm sure you'll agree with me that certain things should never be mixed in between different ranks, for example what Alcide said: Weaponskills. Magic should also stay in their perspective groups. What is the point of having DoM if everyone can use magic? I know it's at a deducted rate, but still, no go. A spell like shock spikes should only be a spell for the enhancer/buffer rank.

Now certain things we can mingle. Now I'm not in tune with all the abilities of the ranks so I'll use old XI terms. Abilities like Berserk, Defender, Warcry, Elemental Seal, Divine Seal, etc were great traits at which you could pass along yet still keep the sanctity of the Job/Rank intact. Only this time instead of a subjob, we obtain these abilities through the ability points we receive.