Mithra is probably the most consistent in 
Debuff Success BoostedDebuff success rate is boosted
buff for all parties. However it cannot completely overturn against debuff failure rate due to the strong emphasis on elemental penalty effects found in Status_Effects#Debuffs. Actually, I tested a same elemental battle (Diablo, Maniac difficulty) with both Mithra and a Cosmos SPEC weapon. So far, it's like 30% ~ 45% in debuff success rate. Not too bad. It sometimes fail or not. The exception is debuff success specialists like 
Danua (Summer). They are unique and rare that way.
It's just my personal opinion that variety can contribute to a lot of game ideas. The permutations can be numerous. I am looking forward to the next star/update of Mithra with a higher debuff rate (in aura), adds lower debuff resistance in call, superior elemental switch buff (see 
Erin's Crystal Wall skill for reference) in call, weak elemental switch debuff in call, maybe some field effect involving debuff success/resistance in call, or simply debuff success (boosted) buff (I don't really see a page for it yet, and they can be in different names as well) in call.
If debuff success rate and superior elemental switch buff is out of the question, then maybe a refresh of foe debuff duration would do. It's like 
Break Boosted (Time)Break mode time is extended
, but with debuffs. For time or turn duration, I don't know, but there's plenty of room to cover (maybe turn as 5th *, but probably game breaking). You don't really need an icon, just a counter name stating otherwise. Alternatively, the same can be done with ally buffs as well. Plenty of permutations, I say.