DRK changes that would make it really fun to play

[Archwizard]

TBN is damage neutral...?

Yes. And what I suggested wouldn't affect damage at all -- the only reward I'm suggesting is a cost reduction that effectively increases survivability, which is no different from people arguing for it to HoT when the barrier breaks.
You're not locked out of using the DA spender if the barrier isn't broken in time, because you can still use it without DA and TBN doesn't eat the resources needed to cast it in the first place.

If you want me to get more specific with what I mean, here's an example to clarify:

* TBN no longer costs MP to activate, but it's on a 25 sec CD like other on-demands. A weaker version of TBN (Shadowskin?) is available at lower levels that has a lower potency barrier and doesn't give DA.
* Shadowbringer has a 30 sec CD and 2 charges, no MP cost but consumes 20% of HP when activated. A weaker version is available at the level TBN is introduced (Flood of Shadow?).
* DA reworked, now causes Shadowbringer/Flood to be cast without an HP cost, and up to two DA charges can be stored at once.
* Abyssal Drain replaces Flood of Darkness/Shadow as the AoE MP spender. Carve & Spit is upgraded to a cone attack with damage falloff on additional targets, and no longer shares a CD with Abyssal Drain.
* Edge of Darkness/Shadow now restores HP whenever it deals damage.

MP is used for draining attacks to heal you just like PLD's spells (which also helps recover from any Shadowbringer overspending), DA remains damage neutral because it's not on an MP cost or refund. You can choose to play conservatively, only using Shadowbringer as often as you have DA, or take calculated risks using the extra charge during burst (which you can quickly recover from with your MP spenders), and how well you thread that needle could become a point of skill for the job at high-end play.

And disclaimer, I want to make explicit here that I'm only defining this example to clarify what I mean, not as advocacy for a specific path the devs could or should take. I'm illustrating just one of many possibilities.