Almost everyone acknowledges that the way of the four Elements Monk is one of the worst classes in D&D 5th Edition. Here’s my attempt to make it useful, while giving it a full “Last Airbender flavor.
Your movements channel primal forces. Way of the Four Elements Monks are half casters with a limited spell list who can cast elemental spells from elemental disciplines you have mastered. The spell list, and class, is inspired by the Last Airbender. This is a replacement for the Way of Four Elements subclass in the PHB and SRD. Unlike the Way of Four Elements Monk, this is a true half caster who gains the ability to cast spells beginning at third level (like the Arcane Trickster Rogue).
When you choose this tradition at 3rd level, you learn magical disciplines that harness the power of one of the four elements. This discipline does not require you to spend ki points each time you use it: the power is innate. The spell progression list is the same as the arcane trickster (spellcasting begins when the player reaches the third level in monk). At third level you choose between one of the four elemental disciplines: Fire, Water, Earth, or Air. You begin with 3 cantrips from the elemental discipline you choose at 3rd level, plus the bonus cantrip from your discipline (Control Flame, Shape Water, Mold Earth, or Gust, depending which elemental path you choose). At higher levels you can learn spells from other disciplines.
Additionally, when you gain a level in this class, you can choose one of the Way of the Four Elements spells from a discipline you know and replace it with another spell from the Four Elements spell lists, which also must be of a level for which you have spell slots. This includes cantrips. The new spell you choose must also be from a discipline to which you have mastered (see elemental disciplines, below).
Wisdom is your spellcasting ability for your sorcerer spells, since the power of your magic relies on your ability to use your intrinsic understanding of an element to conjure and shape it with your movements. You use your Wisdom whenever a spell refers to your spellcasting ability. In addition, you use your Wisdom modifier when setting the saving throw DC for a sorcerer spell you cast and when making an attack roll with one.
Spell save DC = 8 + your proficiency bonus + your Wisdom modifier
Spell attack modifier = your proficiency bonus + your Wisdom modifier
You know the spells in the special spell list. Each of the spells you cast must be of a level for which you have spell slots.
Casting Elemental Spells
To cast one of these spells, you use its casting time and other rules, but you don’t need to provide material components for it. Your casting ability comes from your mental attunement to the elements. All of your spells have a a somatic component, since the way you move your body channels the energy of the elements.
To cast a spell, you expend one ki point per spell level cast. Cantrips do not require the use of ki. Ki points can be used to upcast spells as well. Thus, casting Invisibility (a second level spell) would require two ki points. Upcasting it to make two characters invisible as a third level spell would require three ki points.
The maximum number of ki points (its base ki point cost plus any additional points) that you can spend on the spell is 2, unless otherwise specified otherwise. This maximum increases to 3 at 9th level, 4 at 13th
level, and 5 at 17th level.
Your monk level determines the maximum level of spells you can cast, as well as how many spells you know. Spells, and their levels, are listed below in the spell lists for each type of elemental magic.
As your knowledge of the elements increases, you gain access to other elemental disciplines spells at higher levels. You learn one additional elemental discipline of your choice at 6th, 11th, and 17th level. The elemental discipline you choose gives access to one of spell lists of the Way of Fire, Way of Water, Way of Earth, or Way of Air. From that level on, you can choose to gain spells from those subclasses lists when you gain a level.
Thus, for example, if you chose the Way of Fire at third level, at 6th level you could choose to master water, earth, or air. This will grant you a cantrip associated with the element you choose at 6th level, as well as the ability to choose spells from the new elemental list you picked. By 17th level, your capstone ability is mastery of all four elements, and thus access to all four spell lists.
When you reach a level in this class that grants the Ability Score Improvement feature, you can replace one cantrip you learned from this class’ spellcasting feature with another cantrip from the any elemental monk class spell list you have access to via elemental discipline subclass feature
The Way of Fire spell is focused on striking and damage dealing with fire and light-based spells. The Way of Water is focused on mobility, water, acid, and ice-based magic. The Way of Earth spells focus on protection, battlefield control, and physical damage. The way of air focuses on mobility, stealth, and speed, with lightning and thunder-based damage.
You automatically learn the cantrip Control Flames when you gain access to the Way of Fire Spell list. You do the same with Shape Water, Mold Earth, and Gust when you gain access to the Water, Earth, and Wind spell lists respectively.