Just out of interest, why havent we had an HP Caster since Aatrox?

Aatrox was released in mid 2013, nearly four years ago. There's a lot of interesting design space that I think the Champ Design and/or Champ Update team could do with a health-based caster (considering some of the stuff they've produced recently it could also be great), especially around health-regain mechanics, but the only really interesting example of anything resembling that is Zac's health-cost/health-regen in using abilities and picking up bloblets.
Elsewhere, you've just got fairly outdated kits that could really all benefit from minor GUs to full VGUs, with heals that trigger either on damaging enemies (Vladimir, Aatrox, Mordekaiser) or just by pressing a button (Mundo). While they're fulfilling theme-wise, they're...lacking. They could be much more interesting, or at least feel more interesting to play with/against. But I'm getting distracted, this isnt what I'm here to talk about.
We havent had a HP Caster in over three years. Four of them are just variations of "tanky-magic-damage-dealer", and Aatrox is...well, Aatrox. There's differentation to be sure, but they all fulfill the role of a tanky frontliner, with varying amounts of mobility and utility. What I'm curious about - if not straight up asking/hoping for - is that we get more HP Casters, but that they're also wholly different from what we already have.
I want a HP Caster Assassin, a HP Caster ADC, a HP Caster Support. A HP Caster with no innate healing mechanics and instead is based around maximizing their own damage/utility as they slowly kill themselves, a self-destructive time-bomb. A HP Caster who uses their own life as a resource to lock down and ensnare foes at range.
Health as a resource is interesting, given that your ability to stay alive and your ability to perform are linked to the same resource. It changes the way you play, the way you think about making plays. I'm biased for sure, I love health-casting both as a gameplay mechanic and as a fantasy. I love the thrill of watching health bars shift with casting abilities, rather than just with taking damage. I adore the concept of barring yourself from using a certain ability not just because you don't want to push the wave, but because you don't want to slowly kill yourself doing so. I love being punished with a slash to my health bar for missing a Cleaver or messing up a cast of Tides of Blood, and in turn being rewarded for hitting those things with a drastic change to my enemies hp.
Just my thoughts.
TL;DR - Current HP Casters need visual/gameplay work across the board to varying degrees and are a bit same-y. I would love to see what Riot's current team(s) could do with trying their hand at different breeds/classes of HP Casters.
, who is the HP caster support that I absolutely hate). They have this inherent feast/famine problem because they all need some kind of extreme HP regen to be healthy. And that either falls flat or becomes oppressive, based on how ahead/behind they are.
in that direction. The reality of knife fighting is self-injury, frankly, and that could have been a cool theme to explore on an assassin. Again, though, he would need some kind of regen for that to work, and I'm not sure how healthy that could ever be on an assassin.
HP casters you say...