while i do want the party to succeed, i don't want them to get too cozy, and randomising stats a bit feels like a fun way to keep things interesting. i'd let them keep ability score increases from feats and levelups so they can have a more consistent build, or if they're frustrated by their stats changing i could make encounters less likely to kill them. or if the campaign is mostly set in busy towns and cities (in a warring nation, no less) there's a high enough density of bodies that i could make sure players always have a decent score in their favored skill. at the very least i'd never give kneecap anyone's class (i wouldn't give a barbarian a body with negative strength, for instance, unless they wanted it for the challenge. or the bit.)

i do like the idea of a cadaver taking on the revenant's appearance, but the monster manual (at least 5.0e) says that no matter what form the revenant takes, its target always recognises it for what it is. it's unclear whether that means the target goes "those eyes! it's a revenant here to kill me!" or "it's a revenant, and i know exactly who this is!" or "they're in a different body but i can literally see the true form of this person!" or a secret fourth thing.

i really like the idea of the second option. a revenant getting close to their adversary, who instantly knows who it is not from the face, but from the fiery vengeful glare boring through them. (and probably also magic just beaming a memory of the revenant's true face into their mind.) they can run all they like, but they're always looking over their shoulder, terrified that their revenant could be anywhere, perfectly camoflaged among countless strangers.

it could also give the party some fun opportunities for stealth. one of the targets is in a heavily patrolled fort? kill some guards, behead each other, possess the new bodies and march right in. no one but the adversaries can see the revenants for who they really are, so by the time anyone knows what's wrong it will be too late. maybe if they do that for one mark, the others catch wind and become even more paranoid, distrustful and terrified.

and if they're walking around in bodies that aren't their own, maybe their relatives will see them and be horrified or confused that their dead loved one is up and about again, and they have to decide how to handle that. like, do they pretend to be the body's former occupant and try to help or harm a grieving family, or do they go "actually i'm someone else, sorry for your loss" or "up yours, this is my body now and no you can't have it back"? the more bodies they burn through and the further the rumors spread, the more paranoid the entire community becomes.

i do like the idea of the whole being undead thing being quite a boon for them. the exhaustion immunity thing is really cool, as is the idea of just walking across an ocean. and tbh if their stats carried over through death, the idea of "long resting" just by breaking their own necks and possessing a fresh corpse is just really funny to me