The Dead Land
Optional Rules and Handy Charts
We want to mess with the rules enough to keep up a sense of danger and to make the players feel like they're often in a tenuous situation.
This isn't going to be a horror game, exactly, but it will be bleak enough that you might want to take inspiration from the madness rules on p. 259-260 of the DMG. Would be interesting to have characters accumulate flaws and other character traits over time as they try to deal with the hardships of their adventuring. You may wish to use the Fear and Horror rules on p. 266 to represent the stress of facing overwhelming odds or experiencing despair and loss. When thousands of walking corpses swarm through the city streets, characters might have to make Wisdom saves to stand and fight and probably sacrifice themselves in the process. When they encounter some hideous amalgam of reanimated flesh or find the inhabitants of their favorite village butchered, they may experience horror and find themselves unable to deal with the burden of what they've witnessed.
To enhance the fear of death and the sense of death's constant presence (tying into the themes of the campaign as a whole), use the options for healing and rest found on pp. 266-267 of the DMG. Healer's Kit Dependency means that characters need to keep track of healing supplies (hopefully adding a bit of extra tension on long adventures, and not just more bookkeeping). The Slow Natural Healing rule means that characters do not automatically regain hit points at the end of a long rest – instead they recover Hit Dice at the end of a long rest as usual, and then must spend those Hit Dice to recover hit points.
Use the Injuries rules on p. 272 to keep the tension of combat high (along with the consequences for failure). Consult the custom Lingering Injuries table whenever a character drops to 0 hit points. Remember that hit points represent a character's overall endurance in a fight, and that the hit that puts them down is usually the first direct and really dangerous injury they sustain.
Offer up the Action Options on p. 271 (except for Mark, which should be covered by character-specific abilities) to make sure that your players keep other dramatic options in mind during combat.
It might be worth looking at the Gritty Realism rest option (p. 267) again, but this is still going to be a combat and adventuring-focused game, so the Slow Natural Healing option might be enough to acheive the same effect of making every lost hit point pretty painful. We still want the players to be heroes, though they may be up against great odds and might not always be able to be heroic.
Massive Damage (p. 273) does make combat more brutal, but might just be another rule to remember and doesn't really square with our understanding of how hit points should be understood narratively. Characters should be heroic enough to sustain a number of small wounds before succumbing to a direct hit that drops them.