I liked the Innistrad block of M:tG so very, very much, and I loved Plane Shift: Innistrad. There's just one really important thing from Innistrad that it didn't have: the Geist-Honored Monk. As it happens, I don't know the M:tG canon for this card, but I found it fascinating. What follows, then, is my impression of what they might be about, in the Way of Lost Souls monastic tradition.
Way of Lost Souls (Monk)
The monks of the Way of Lost Souls have an unusual approach to ki, as they share a portion of their life force with the souls of the dead that wander the world, so that these ghosts may find their way to their final rest.
Some souls are benevolent, some malicious, and others simply adrift on ethereal winds. It is not unusual for staunchly altruistic monks to establish a bond with a evil soul, acting as its caretaker and preventing it from becoming something like a specter or wraith. Evil monks often find benevolent lost souls tedious, but the practical benefits of such a pairing may outweigh that consideration.
When you choose this tradition at 3rd level, you forge a bond with a lost soul: a person who has died, but has not passed entirely into whatever afterlife awaits them. The lost soul does not fully manifest in the world and cannot be affected by attacks or spells under normal circumstances. When you use the Attack action, one attack may target a creature up to 30 feet away from you with a melee weapon attack that deals the normal amount of damage for your unarmed attacks, including magical bonuses if any. When you gain this feature, choose whether the attack deals bludgeoning, necrotic, or psychic damage. You may choose to shove with this attack, but not grapple.
Spells granted by the Way of Lost Souls use your ki save, and do not require material components. Your spell attack bonus is your proficiency bonus + your Wisdom modifier.
If your lost soul deals bludgeoning damage, you may spend 2 ki points to cast bless.
If your lost soul deals necrotic damage, you may spend 2 ki points to cast inflict wounds.
If your lost soul deals psychic damage, you may spend 2 ki points to cast command.
At 5th level, you can spend an extra ki point to cast your spell as if you had spent a 2nd-level spell slot. At 9th level, you can spend two additional ki points to cast your spell as if you had spent a 3rd-level slot. At 13th level, you can spend three additional ki points to cast your spell as if you had spent a 4th-level slot. At 17th level, you can spend four additional ki points to cast your spell as if you had spent a 5th-level slot.
Two Souls' Bond
Starting at 6th level, you gain a feature based on the type of damage your lost soul deals.
If your lost soul deals bludgeoning damage, whenever you hit with a melee attack, you may spend 1 ki point to deal an additional 1d10 radiant damage. Further, you may cast beacon of hope without expending a spell slot; once you do, you must complete a long rest before doing so again.
If your lost soul deals necrotic damage, you gain resistance to necrotic damage, and whenever you hit with a melee attack, you may spend 1 ki point to deal an additional 1d10 necrotic damage.
If your lost soul deals psychic damage, you gain resistance to psychic damage, and whenever you hit with a melee attack, you may spend 1 ki point to deal an additional 1d10 psychic damage.
Tempest of Souls
Starting at 11th level, as an action, you may spend 4 ki points to fill your lost soul with power and fury. It seems to be everywhere at once, striking all enemies who draw near. When an enemy approaches within 15 feet of you, or starts its turn within 15 feet of you, it must make a Wisdom saving throw. On a failed saving throw, it takes 3d8 damage of the type your lost soul deals, and its speed for the turn is halved. On a successful save, it takes half damage and no further effect. This effect requires your concentration and lasts up to 10 minutes.
Starting at 17th level, your lost soul can attempt to possess an enemy or a dead humanoid body. Choose one humanoid within 15 feet of you whose Challenge Rating is less than your level. It must succeed a Charisma saving throw or be possessed by your lost soul. While your lost soul possesses a target, you may not use your Lost Soul or Tempest of Souls features. Your lost soul now controls the body, but does not deprive the target of awareness. The possession lasts until the target drops to 0 hit points, you end it as a bonus action, the lost soul is forced out with an effect like dispel evil and good, or 10 minutes elapse. The possessed creature uses the host's statistics, but is immune to the charmed and frightened conditions. It does not gain access to the target's knowledge, spells prepared, or spells known.
Alternately, choose one humanoid body that has been dead for 1 hour or less. Your lost soul possesses it, and while your lost soul possesses a target, you may not use your Lost Soul or Tempest of Souls features. Your lost soul now controls the body, which has hit points equal to your maximum hit points. The possession lasts until the target drops to 0 hit points, you end it as a bonus action, the lost soul is forced out with an effect like dispel evil and good, or 10 minutes elapse. The possessed creature uses the host's statistics, but is immune to the charmed and frightened conditions. It does not gain access to the target's knowledge, spells prepared, or spells known. The creature is alive for the purposes of all spell effects, except those that restore the dead to life.
If you wish, your lost soul can give control of a dead body back to the body's original soul for the duration of the effect. If you do so, the creature has full access to its knowledge, spells prepared, and spells known.
Once you use either form of Ghostly Possession, you must complete a long rest before you can use it again.
I strongly recommend that DMs and players work together to give the lost soul a name, personality, and history. The lost soul probably isn't available for conversation 100% of the time - possibly only when no one else is within earshot. Lost souls that deal bludgeoning damage are generally benevolent, and may be laboring under some kind of curse that prevents them from finding their way to the afterlife. Those that deal necrotic damage are probably right on the verge of becoming specters, wraiths, or other malicious, incorporeal undead, driven by hatred and hunger for mortal warmth. Those that deal psychic damage have probably forgotten much of who they were, and there might be side-quests to learn their secrets.
I didn't write this into the rules, but personally I think it would be fine to allow a character to switch from one kind of soul to another, probably only when they level up. This could involve doing enough for a lost soul that it passes on to its afterlife, so the monk bonds to a new one - the story of this monastic order certainly admits the possibility of resolving many souls in the course of one's career.
I struggled with even finding a core mechanical idea (beyond "a ghostly presence does some shit") for about two weeks. One of the earlier ideas that partially survived into this version was taking the Elemental monk and changing out all of the wizard spells there for cleric spells, but between OGL limitations (that I'm adhering to for a future publication plan) and not wanting to push per-short-rest healing, I wasn't really finding enough depth there. There's also the issue that the Elemental monk is widely disparaged for being underpowered.
I moved instead to a good/neutral/evil divide within the class, reflecting the lost soul rather than the monk. I feel like that could be an interesting, fun story, much like the Haunted fighter that I wrote for EN5IDER last year (and will be re-releasing when the copyright reverts to me in a month). Three subclasses within one class is... not revolutionary, but not universal either. I'm a little worried that bless is too good and command is not good enough, but I don't have a ton of basis for that. Also, command gets a lot better with higher level slots/more ki points.
Tempest of Souls is spirit guardians, but its speed-reducing power is moved from always-effective to only on a failed save. Stands-in-Fire and I may spend a lot of time talking about this spell.
Ghostly Possession is the most complicated part of the subclass, but it should still be a bit simpler than a ghost's Possession feature, which requires more recalculation. It can also be used as a short-duration combat rez, with the interesting use case of letting a cleric or paladin revivify herself (since she is still a dead body and thus a valid target, but also able to cast spells at the time). Ghostly Possession turns off your Lost Soul and Tempest of Souls features to support the fiction that it's your lost soul possessing or animating the creature.
And yeah, I know that the Geist-Honored Monk enters the battlefield with two 1/1 white Spirit tokens. That would be been a nightmare to handle in presentation, so I went with one (that might be white, blue, or black) instead.