If you want a really cool setting to adapt to, The Late Bronze Age Collapse is incredible. You can also use a lot of Greek , Egyptian, Assyrian, etc... mythology to flesh it out with magic. Or go a few hundred years forward into the Iron Age and the Achaemenid Empire, or rise of Neo Assyria too...
Twilight had a really good character creation system that took on terms of a career to build up skills and attributes. So you could be a highly skilled 50 year old past your prime and have effectively a good spread of skills but then age modifiers on attributes. Or you could stack it so you were...
Yeah, it absolutely affects the game feel. If it's a lethal system, a death spiral actually acts like a buffer to sudden death, but in a a D&D style game, it doesn't really serve as much of a purpose.
I know there is often hate for death spirals, but I like them when they are done right. Nothing screams Get the hell out of dodge like an ever failing ability to perform.
Where I think they go wrong is where they hinder your ability to mitigate further harm and/or escape.
The Gorn are pretty nasty in Beta canon too though. They only start to become more civil with other races once they realize that they are sentient and sapient too. In all reality, there isn't much room for new in a prequel, so changing p the Gorn, Tholians, Husnock, etc... is acceptable to me.
I kind of made a hybrid system where hit points counted to wound level thresholds. You could absorb up to your Hit points in damage before you took an injury. hit points healed every round, but wounds took increasingly longer and applied penalties. Twice your HP in damage in a round caused...
I think if anything kills D&D it will be D&D itself. It nearly died with 4e and that allowed Pathfinder to have it's glory days and overtake it. If 5e was a bad product that didn't attract back the old guard gamers and gain popularity through the likes of Mercer... I think we would not see any...
I read in a 2e book once that the Drow were once High Elves who through worship to nd torture from Lloth took on a visage and characteristics that were perversions and opposites of their former selves. This is why their hair turned white and their skin turned black as coal.
Also S-HAR-K Rounds
Selachimorpha Heavy Assault Ready Kern rounds - perfectly designed to be fired from any standard Mauser Rifle.
Our popular SHARK rounds will never be jumped.
The Fallout universe has the Ripper.
Its essentially a purpose built chainsaw sword. even in universe, there are far better melee weapons and the Ripper is a signature fallout weapon like the Powerfist and Junk Launcher.
In all, even IP that have stuff like this don't really take it seriously...