Winter Walker Ranger

 Icewind Dale: Rime of the Frostmaiden is one of those published adventures I'd be really curious to run - it's a big sandbox with a horror theme, a frozen wasteland in perpetual darkness (in the name of clarity, the maps for the adventure are all bright and easily legible, but I think it's an important thing to stress to the players that it's all profoundly dark all the time. True Detective: Night Country is a good reference to consider for the level of dark, frozen spookiness.

Anyway, this subclass feels built for such an adventure and environment: a decidedly cold-themed ranger subclass. Let's see if it's any good!

Winter Walker Spells:

1st: Ice Knife

2nd: Hold Person

3rd: Remove Curse

4th: Ice Storm

5th: Cone of Cold

    Remove Curse is good to have when you need it. I'm not sure any of these spells are going to be all that great for you as a Ranger. While Cone of Cold is good as a big-area AoE spell, by level 17, it's not going to feel as powerful as when the 9th level Sorcerer cast it half the campaign ago.

Level 3:

Frigid Explorer:

You gain a few benefits:

Biting Cold: Damage from your weapons, ranger spells, and ranger features ignore resistance to cold damage.

    Cold resistance is certainly more common than, say, Necrotic resistance. Indeed, this will help not only when fighting traditionally chilly foes, but also I believe most demons and some devils. Of course, your weapons aren't going to deal cold damage by default, but let's keep looking.

Frost Resistance: You gain resistance to Cold damage.

    A reasonably common damage type, but also very useful when exploring in cold regions, where you'll be able to skip cold weather gear and happily cross the tundra in a t-shirt.

Polar Strikes: Once per turn, when you hit a creature with a weapon, you can deal 1d4 extra Cold damage. At level 11, this goes up to 1d6.

    It's not a ton of damage, but it's always there for you. You'll get more out of this if you can make lots of opportunity attacks or a friend has ways to let you make attacks as a reaction.

Hunter's Rime:

When you cast Hunter's Mark, you gain temporary hit points equal to 1d10 plus your Ranger level. While a creature is marked with Hunter's Mark, it can't take the disengage action.

    So, this is going to be a class that strongly encourages you to use Hunter's Mark. That carries with it all the problems we've talked about when designing a class around a 1st level spell that requires concentration. But these effects are both pretty decent - we get a lot of uses of Hunter's Mark, so we'll be able to get a lot of temp HP. Also, by preventing disengages, we get more opportunity attacks!

Level 7:

Fortifying Soul:

As a magic action, you can choose a number of creatures you can see equal to your Wisdom modifier (minimum 1). Each creature regains HP equal to 1d10 plus your Ranger level and has advantage on saving throws to avoid or end the frightened condition for 1 hour. You can do this once per long rest.

    This is a decent little group heal - notably with no range limit beyond being able to see them. When you get this it's going to be an average of 12.5 HP - not a huge heal, but it can give you some breathing room. The fear protection will be situational, but can be good.

Level 11:

Chilling Retribution:

When a creature hits you with an attack, you can take a reaction to force them to make a wisdom saving throw, and on a failure, they are stunned until the end of your next turn, with a speed of 0 (remember that stunned creatures can move now - though not in this case). You can do this Wis times per long rest.

    I think the biggest downside is that it's most likely going to go off on their turn, meaning that they've already gotten to do something, and it'll end before their next turn, meaning that while you might cut off additional attacks and perhaps a bonus action or movement (and reactions,) you aren't robbing them of a turn the way you would with a Stunning Strike or the like.

Level 15:

Frozen Haunt:

When you cast Hunter's Mark, you can adopt a ghostly form until the spell ends. You can do this once per long rest, or expend a 4th or higher level spell slot to get another use. You get the following benefits:

Frozen Soul: You have immunity to cold damage. When you first adopt the form and at the start of each of your turns thereafter, each creature of your choice in a 15 foot emanation takes 2d4 Cold damage.

    While an average of 5 damage per turn is pretty low at level 15, it is both AoE and guaranteed damage.

Partially Incorporeal: You are immune to being grappled, prone, or restrained. You can also move through creatures as if they were difficult terrain but take 1d10 force damage if you end your turn inside a creature or object. If the form ends while you're inside a creature or object, you are shunted to the nearest unoccupied space.

    While the incorporeal movement has some potential, I think the immunity to those conditions is a bigger deal - pass right through the Tarrasque's stomach. Using this purely for utility outside of combat might, however, require you to do something silly like putting Hunter's Mark on an ally (not great as that will prevent them from disengaging, though you can drop the spell if combat breaks out). As usual, I assume that if you aren't given a hovering fly speed with this, somehow the floor will still hold your weight despite being incorporeal.

Overall Thoughts:

I honestly like the flavor of this - while the art for the subclass doesn't really give off this vibe, I can see this being a really spooky ranger, and it'd be quite appropriate for Rime of the Frostmaiden (though note that a lot of creatures in that are probably fully immune, not just resistant to cold damage).

I also think that this subclass probably leans a little more toward a melee build, though I also think 2024 Rangers are probably better off going that direction anyway.