I’m starting a Wild Beyond the Witchlight campaign but I’m letting my players make up whatever they want for what got stolen from them.

One player asked if he could have lost his tangibility, and functionally play as a ghost. I’m willing to let him if I can figure out a balanced way to play it. Any ideas?

You could make it so most natural terrain and structures count as a full turn of movement to pass through, or only allow them to not be able to see through walls so he would effectively be wandering blind and not know if they are just wandering off towards the center of the world.

It's certainly a fun idea. But it may be difficult to balance. I would start by reminding (warning) the player of just what they would be giving up to be a ghost. Being incorporeal can be useful, but it has certain drawbacks as well. For starters, you can't touch or manipulate any physical objects! That's kinda big. You can't wear armor. You can't wield weapons. Okay, so maybe they play a spellcaster. But they still can't pick up or use any magical items. They can't drink potions. Being incorporeal is complicated. 

Also, they wouldn't have all the abilities from the Ghost stat block, at least not right away. Ghosts can cast Etherealness, a 7th level spell, at will. That's a bit much! They also have a ton of resistances and immunities. They can fly. They've got multiple powerful magical attacks. That's way too unbalanced. So if they really really want to play as a ghost, I would say maybe they just died, and they just came back as a ghost, and they haven't quite yet figured out how all their ghost abilities work. They're still just a probationary ghost, a ghostly intern, a ghost trainee. 

So what would that look like? Well, let's try to keep their power level balanced with the other available species: 

Darkvision 60'.   Resistance to necrotic damage.   Vulnerability to radiant damage.   Fly speed of 30' per round.   Can't be grappled. 

Ethereal Sight: You can use an action to activate this ability. For the next minute you can see 60' on the ethereal plane. You can use this ability a number of times per day equal to your Proficiency Bonus.

Ethereal Movement: (this is the tricky part): You can move through creatures or objects as if they were difficult terrain. If you end your turn inside their space, you take 1d10 force damage. BUT, you can only use this ability for one round at a time, a number of times per day equal to your Proficiency Bonus. Maybe (maybe), they can regain one use per short rest, but even that is pushing it. 

The downside is, again: you cannot touch, manipulate, or use any physical objects. 

It's not perfect. It'll need some tweaking to keep it balanced. But it's a start.

Greetings CaptainCleverUsernameHere,

Within Van Richten's Guide to Ravenloft, there are three Lineages which effectively are Subclasses.

One such Lineage is called the Reborn which are basically Ghosts that you can play as a character.

In the current campaign I am playing in, my Character started as a Tabaxi Fighter, but was attacked by a Vampire, was injured and got away and was not fully turned, so they became a Dhampir, so now I am playing as a Dhampir Fighter.

Hope this helps...

Cheers!

Quote from AnzioFaro >>

So what would that look like? Well, let's try to keep their power level balanced with the other available species: 

Darkvision 60'.   Resistance to necrotic damage.   Vulnerability to radiant damage.   Fly speed of 30' per round.   Can't be grappled. 

Ethereal Sight: You can use an action to activate this ability. For the next minute you can see 60' on the ethereal plane. You can use this ability a number of times per day equal to your Proficiency Bonus.

Ethereal Movement: (this is the tricky part): You can move through creatures or objects as if they were difficult terrain. If you end your turn inside their space, you take 1d10 force damage. BUT, you can only use this ability for one round at a time, a number of times per day equal to your Proficiency Bonus. Maybe (maybe), they can regain one use per short rest, but even that is pushing it.  

 This is great! Putting it into comparison with existing species’ power levels was super helpful, and balancing ethereal movement is a good idea

Quote from CaptainCleverUsernameHere >>

I’m starting a Wild Beyond the Witchlight campaign but I’m letting my players make up whatever they want for what got stolen from them.

One player asked if he could have lost his tangibility, and functionally play as a ghost. I’m willing to let him if I can figure out a balanced way to play it. Any ideas?

 Yup,

1. reduce his maximum hit points by 1/2 and give him resistance to fire, cold, acid, lightning, thunder, necrotic, and non-magical bludge/slash/pierc, and immunity to Poison & the Poisoned condition.

2. Give him immunity to being Grappled but also unable to make Unarmed Strikes/Grapple/Shove.

3. Give him the Ethereal walk 1/SR as mentioned above.

This should be relatively neutral in terms of power, so can be added onto any existing character.

So, there is a YouTuber by the name of Pointy Hat, and he has already addressed this by making a ready-to-use race that is a ghost called the Phantasm

Phantasm Race