I love this idea. Maybe the extra augments you gain on your weapons be gained by somehow sacrificing different Rare/Ex items from the game to come up with different augments randomly taken from that item depending on the level of your weapon.

Let's say level 85 Empyrian weapons can only sacrifice Rare/Ex items up to level 60 to try to get their augments so we'll take a level 85 Twashtar as an example. You might be a career Bard and want to augment your Twashtar to have String Magic on it.
You'd take and you would have a chance to get HP+13, VIT+3, String Instrument Skill +3 or Emnity -1.

I feel like this would make E/R/M weapons relevant and the rest of the game content at the same time. I mean come on guys, imagine Occasionally Attacks 2-3 Times from a Ridill on your Ragnarok. =D