They have already done set bonuses with lower level GC gear, and with the sanction granting and sanction item (again GC lower level gear and item). But yes, only affects main stats.

Maybe with the expansion (or sooner though doubtful) they could add such a thing (item or whatnot) that boosts job specific actions? Could still tie it in with GC to keep that theme going, but make it a tank/DPS/healer enhancement item, boosting tank/DPS/healer abilities (shorter CDs, stronger effects, more potent attacks, etc). But specifically affecting skills as opposed to stats (or really it would just be another fancy item aside from but no different than food).