I think a few different kinds of sub stats would be nice, to allow more variation between items of the same level. Including some stats that don't just make you deal more damage....

Ideas that come to mind; 'Stamina' - increases your base TP, and/or increases the natural TP recovery rate.

Constitution - reduces physical damage taken, increases your natural HP recovery rate, and reduces the duration of status ailments

Wisdom - reduces magical damage taken, and increases MP natural recovery rate (Or reduces the MP cost of spells instead, as that would probably work better with how BLM's work)

A stat that slightly extends the duration of Cooldowns and/or reduces their recast timers. (Can't think of an appropriate name though!)