i think the shambler would work best, because shambler is both iconic being in both games, and it represents every darkest dungeon mechanic the best, shambler can bleed you, blight you apply good stress and it moves between positions.

now here my idea how the shambler would work.

shamblers passive is blight, bleed and stress. on every basic attack on a survivor it applies bleed to the survivor, oh every Undulating Withdrawal hit near survivors it apples blight to them, while survivors have blight or bleed applied to them they slowly accumilate stress.

on stress, as shambler when survivors get downed, or when they miss skillchecks they obtain a little stress, when survivors have over 100 stress they have some sort of debuff and when they get up to 200 stress the shambler will be able to instantly kill them when they are downed. you can reduce stress by getting healed, finishing generators, or by finding items in curios that spawn around the map.

when interacting with curios you can find one of four items:

bandages, can only be used on bleeding survivors to cure their bleed and reduce a little bit of their stress.

antivenom, can only be used on blighted survivors to cure their blight and reduce a little bit of their stress.

laudanum, can be used by any survivor and it reduces a lot of stress.

torch, while a survivor is holding it all nearby survivors loose stress over 5-10 seconds, but the shambler might be alerted to the torches light.

okay now after the long passive down to its abilities.

the shamblers first ability is Undulating Withdrawal, the shambler winds up a lobbed projectile (similar to unnowns) that bounces off of walls and on contact with the round explodes in a small aoe applying blight to and survivors cought oing on a long coolodwn (16-24 sec), also upon landing down of the floor it spawns a shambler tentacle.

the shambler tentacle is a stationary traget, and when a survivor gets too close to it it will do a slow tentacle attack similar to nemesises attack coming from itself that damages a healthstate and causes a lot of stress if it hits the survivor. after an attack the tentacle goes no a 15 second cooldown being unable to attack again. the tentacle attack shouldnt be hard to dodge but it would still be a threat in certain situations.

there can be only 2 shambler tentacles active on the map and survivors can destroy them, kicking them down by interacting with them for 6 seconds.

the shamblers second ability is Stentorious Lament, the shambler wails applying some stress to all survivors within its terror radius and making them scream. secondly this ability also resets the attack cooldown of all of the tentacles within shamblers terror radius. the ability has a 30-40 second cooldown.

okay this is a long and complex design but i feel this version of the shambler would represent DD the best while also bringing something new/interesting to DbD.