That's neat for the "normal" builds that rely on ignite, bleed, poison etc but won't do a whole lot for the builds that immunities were designed for.
For a fireball ignite build, you'll ignite and it'll be great and it'll be harder for sure, but a flameblaster will just ignite and gtfo while the boss is dead 2s later.
Same with giving bosses a resolve bar, if it takes a few tries then the one try that works is all that's needed which can be abused very easily, or any sort of internal cooldown.
Don't get me wrong, there's a lot of cool potential for some of the interactions that people mention like enrage or clickable barrels, which helps the little guy but does basically nothing against the main offenders. Nerfing the offending skills is not the best route either, because some skills are fine but outside interactions push them to questionable degrees even if those interactions aren't alone a problem.
They're..not punished. If you mean builds that have ignite that is a small boost to damage but isn't like a detonate dead or flameblast, sure those are hurt but not really as much as the above (which tend to be the abuse cases for discussion's sake) and are better at dealing consistent damage. Everyone complained about BF mines trivializing uber, now the complaint (granted it's been "adjusted") is that they can't trivialize all these guardians. So as will be brought up again in a second, if you have a better solution we'd love to hear it.
Well that's the crux, if you have any suggestions that work no doubt everyone would love to hear it. Split phases really do nothing against that tbh, Main Atziri isn't ignitable, split ones are, few ignite builds do (well, uber) atziri.
So again, yes the current complaints are about immunities, but most of the interesting changes don't really fix the issue because there are such a wide variety of builds that use these mechanics. A voidheart jugg will lose some damage but not really care that hydra is poison immune, a Path of Exile chaos barrage or spectral throw will be able to kill hydra but will have to actually play the game, and a bladefall miner (for discussion's sake) can't kill hydra. If you remove immunity or add a lever to press suddenly bladefall trivializes (yes i know it's been nerfed) hydra in an instant, spectral throw has a comfy time, and jugg doesn't have to swap a ring. This is the simplified case and the reality of managing every single fire skill and every skill that can apply bleed, poison, etc is a lot to deal with. Even the freeze suggestions of giving resolve will find a way to break it unless there's a long enough icd that an ignite build would trivialize it anyways.
That last point is certainly interesting, though there would surely be a ton of complaints from the "little guy" again because if it's dangerous to punish the abuse cases mr 4k life fire trap has no hope.