It seems like the problem there is that CDDA’s difficulty is heavily front-loaded, and by the time you get a character to end game there isn’t a reason or desire for more plusses at the beginning of the game[1]. The first time you play you aren’t really sure what you need to do and you get murdered by zombies, wolves, and/or a moose. You haven’t been able to kill a thousand monsters or gather any NPCs or really do anything of note. Once you learn how to handle those things, the early difficulty isn’t that bad and you can (within a few weeks) bootstrap to the survivor-armored midgame, and once you hit that the transition to late-game (invulnerable cybernetic mutant with a deathmobile) is entirely voluntary - you can go to labs, towns, etc. at your leisure and fully prepared, and the sorts of things that will kill you there will a) only tend to kill you once, since you’ll prepare for them after that and b) by their nature tend to kill you before you can react, which means that 1% extra health or whatever wouldn’t help. A ‘successful’ CDDA run is also long enough that I think I’d get annoyed if there were things or challenges that I couldn’t reasonably take on without having ground out several successful sessions previously.
You can also already sort of do this by starting a new character in the same world - I had a late-stage character die an unexpected death, started a new character, and after the usual early-game scrabbling made my way to the old character’s stash and helped myself to the books, CBMs, armor, and crafting materials that I had left there, basically removing a lengthy period of scavenging.
If something like NG+ was implemented, about the only way that I could see it happening would be to combine it with the suggestion about making things better: Take a character and opt to retire them, and the world gets a bit ‘better’, influenced by their stats and companions and how far they have gotten in the faction quests, or something. It could modify spawn rates in the area around where they retired, maybe build some buildings like the commune does, something like that. You would have the usual new-game-in-existing-world issue that all the perishable food has spoiled, but you (as a player) can probably deal with that…
[1] Except maybe QoL things like self-aware, robust genetics, and so on that experienced players normally opt out of because they can get them later or don’t need them