The Blades Hack - Dark Deals
Dark Deals are one of the mechanics that feels like it'd be hard to go back to BitD with -- kind of like how, in playing other games, I end up missing flashbacks.
The way Dark Deals work is when the GM gives you a consequence, you can suggest your own, and if the GM finds it more interesting / impactful, then that replaces the original consequence (and cannot be resisted).
This kind of collaborative consequence-building is super inline with BitD's ethos. I actually had the idea after reading about a about a newbie GM who didn't feel comfortable with improvising consequences. Having a mechanic to allow players to do it felt like a no-brainer, especially since BitD's existing "incentivised masochism" mechanic in devil's bargains, to me, has some issues I don't love (DB negotiations can take ages, and +1d is quite minor).
One more interesting element of Dark Deals is that taking a trauma is always accepted. I often found that in BitD, there's not much of a threat of death. In TBH, the threat is significantly more present, but the PCs are also given some tools to deal with it. Taking a trauma for a dark deal can literally save your life, but since the max number of traumas have been brought down, it's something you can do twice.
So far it's one of the mechanics that's been a good success. It gives players more authorial control and allows for more collaborative storytelling.