Again, I don't think TBN's exclusively at fault. I just think its current design, particularly around Dark Arts, is an obstacle. It would be more accurate to say it's an issue of the Darkside abilities as a whole, but TBN is presently balanced around them as its counterattacks, which reinforces the issue.
That's a marginal difference though, and only adds buttons for the sake of buttons; at best you find yourself alternating which of the two you use in the rotation every thirty-ish seconds, and the choice isn't particularly meaningful since you're still focused on which does optimized damage, particularly based on what's compatible with Dark Arts.if we're talking about adding more MP spenders to spice up the kit then it's more how to make sure Edge and the new MP spender don't have a clear winner in every situation. Two ways I can think of is a) new spender does more damage but doesn't add Darkside and b) new spender has a CD. I think it'd be possible to add both of those with a 3K MP cost and they'd fit in perfectly fine with TBN. I can't think of a reason for them not to cost 3K MP but if they do need to then limit Dark Arts to only working on Edge/Flood.
If you compare it to the days of original!Dark Arts granting increased healing or enmity or causing attacks to inflict Blind, as well as the potential growth into other sources of mitigation and utility -- tools that actually expand our gameplay and ability to react situationally -- our immediate use of MP for almost exclusively damage purposes and to limit TBN remains pretty narrow, even in that scenario.
Right now, you can't really diversify our use of MP without adding buttons that are all basically equal (or only situationally superior) in damage value; any less and you won't use them, no matter what other utility they may provide. Even having them be buttons that consume variable amounts of MP has to be weighed against you spending 3k to activate TBN to get Dark Arts; if it costs more then bursting TBN's barrier becomes a damage imperative, and if it costs less then spending Dark Arts on them becomes an inherent damage loss.