I went through bane's responses, and here are some ideas for currently unused responses
1. Bane has several anger/failure lines which are way too good to stay unused. I assume the failure lines were originally ment for allies canceling Nightmare, but that woudn't work well. It was hard to think of something for them, but here is one idea:
Bane has a powerful spell which requires channeling. So how about Bane getting annoyed when he gets interrupted? The criterion would be to get stunned/silenced/interrupted by the enemy while channeling Fiend's Grip.
Sand King has similar responses upon getting his Epicenter canceled.
The following lines could be used for this:
bane_ability_failure_03 "Hhoh no!"
bane_anger_03 "A vish yoo stavin arra ah ah ah!"
bane_anger_04 "A rishnu arh!"
bane_anger_05 "Om mo mo ar a ya ya ya!"
These two could be used when interrupted by allies, instead of enemies (like getting astral'd)
bane_ability_failure_01 "You let them get away!"
bane_ability_failure_02 "Fool's error!"
And these could be used when Fiend's Grip gets blocked by Linken's Sphere
bane_anger_06 "Wurra ayav!"
bane_anger_07 "Veer aies!"
bane_anger_09 "Arooma mut!"
2. Bane has several unused attack responses which could be used as Enfeeble and Nightmare cast responses, since they thematically fully fit those spells. And the same time, Enfeeble has a couple of lines which are kind of weird for the spell.
These responses could be repurposed as deny responses, away from Enfeeble cast. They are kinda random and don't have anything to do with Enfeeble.
bane_ability_enfeeble_04 "Nice try."
bane_ability_enfeeble_05 "Better luck next time."
This response could be repurposed as a Brain Sap cast response, away from Enfeeble cast. Enfeeble doesn't give Bane anything in return, it only weakens enemies. Brain Sap, on the other hand, does.
bane_ability_enfeeble_06 "Your power flows to me!"
As replacement, these attack lines could be used as Enfeeble cast responses. The ability literally weakens enemies, so these make full sense here. He even already has a similar Enfeeble response like this.
bane_attack_05 "So weak!"
Finally, this attack response could be used as a Nightmare cast response. Alternatively, it could be just enabled as an attack response. It's not like it doesn't fit.
bane_attack_06 "Your nightmare…"
3. A couple of other unused responses
Bane has some unused rune pickup responses. I don't think they'd be too dificult to understand in-game, especially since runes have a unique sound anyway.
This illusion rune line as a rare pickup response
bane_illus_02 "Myriad terrors!"
And this regen rune line as a rare pickup response
bane_regen_02 "Fear can be healing."
Also, this move line could be used as an attack responses
bane_move_15 "To the fray."