I'm kinda sure that as of today, Diversion is one of the best use of the actual Role Action system, both MCH and BRD should have access to it, because the agro drop of Tactician an Refresh are clunky. Especially if the Bard uses refresh to milk a bit of Foe's Requiem. (In my former team, we had BRD and MCH, MCH used refresh on its opener so the BRD had the two songs available, and she kept ripping aggro anyway). Aggro management is part of playing this game and I don't mind. But I'd rather have all proper tools to do so, and it's a bit unfair for BRD and MCH.
If we are to keep the system as it is, I think having the aggro drop being changed to the same effect of Diversion (less aggro generated) on both songs would be perfect. And include an aggro drop on Repelling shot for BRD, Blank on MCH (and Fluid Aura or Assize on WHM).