Sienna’s new health to cooldown is a great addition and allows Unchained to use their ult more liberally instead of saving it for an emergency button, while still keeping a heavy punishment if you do overheat. I do believe this should be part of the Blood Magic passive though.
My main reason for this is it would allow Unchained players to take Natural Talent or Enfeebling Flames without having to worry as much about saving their ult for a possible overload. Currently an unchained can run enfeebling flames and flame aura for a very nice combo, but players may be hesitant to use their ult just for the aura because no matter how careful you play, stuff can happen and its not hard to overload and explode, especially if you’re trying to play on medium to high heat for the melee bonus. Having the health cooldown exchange as a passive would keep a punishment for overloading, but allow every build to use living bomb as more than just an emergency button, but use it also just for the fire aura, or to give yourself and allies temp HP.
My second minor reason I believe it should be passive is it would fit the theme of blood magic. Sienna exchanges heat to control health damage taken, so i think it would be thematically fitting for another part of it would be exchanging health to control heat.
As for a replacement talent, I’m not exactly sure, but I have been thinking about a talent that would operate a bit like gromril armor. Every 20-30 seconds or so, the next hit Sienna takes will only generate half the amount of heat it normally would. This would allow that talent row to have a good enemy debuffing talent, which is good by itself and a decent help to blood magic, a talent specifically for strongly improving blood magic, and a talent that gives a small but nice benefit to casting focused unchained builds as well as a small benefit to blood magic.