As I mentioned in this question, I have recently started watching the Avatar: The Last Airbender animated series and am inspired to improve the otherwise underwhelming Way of the Four Elements monk archetype. To this end, in addition to the suggestions proposed in the other question, I have been expanding the list of Elemental Disciplines to include other spells that I think suits the themes of airbending, earthbending, etc. (especially the elemental spells included in Xanathar's Guide to Everything, originally from the Elemental Evil module).
For spells like erupting earth, flaming sphere, tidal wave, wind wall, etc - in other words, spells that are within the range of 1st-5th level spells - I can simply copy what has already been done for the existing official Elemental Disciplines with regards to discerning how much ki it should cost to cast these spells.
Excluding those that have their own rules rather than allowing the casting of spells (e.g. Fangs of the Fire Snake), the disciplines that cast spells all cast spells that are in the range of 1st-5th level spells, and all follow the formula of "ki points required = spell level + 1" (except for Rush of the Gale Spirits, which only costs 2 ki but lets you cast gust of wind, a 2nd level spell, but it's a weak 2nd level spell, so that's probably why it's slightly "cheaper" than the other disciplines for casting 2nd level spells).
I think high level earthbenders, firebenders, etc, should be able to do truly Avatar-level powerful bending once they reach tier 4. More specifically, I want to come up with some disciplines that add spells of 6th level+ that a 17th level Way of the Four Elements monk can take, but without this being broken. Spells I'm considering include:
- bones of the earth (6th level spell)
- earthquake (8th level spell)
- fire storm (7th level spell)
- investiture of X (6th level spells)
- move earth (6th level spell)
- tsunami (8th level spell)
- wall of ice (6th level spell)
- whirlwind (7th level spell)
Again, to reiterate, all of these disciplines would be available only to tier 4 monks, meaning they'd all have the (17th level required) prerequisite. Also, at time of writing, I am not currently considering including any 9th level spells such as meteor swarm, so if excluding 9th level spells helps in any way, that works for me.
If I were to include disciplines that allowed the casting of 6th level+ spells, following the formula I derived (so 6th level spells would cost 7 ki, 7th level spells would cost 8 ki, and 8th level spells would cost 9 ki), would this still be balanced? Would the ki cost need to be increased because of the fact that these are "higher level spells"?
Given that ki can be replenished on a short rest, would I also need to add additional restraints on these "higher level spells" such as only being able to cast them once per long rest (like how certain warlock's Eldritch Invocations have that restriction, such as Sculptor of Flesh, even though it still uses a warlock spell slot), or would the ki cost be enough on its own?
By "higher level spells", I'm referring to the fact that 6th-9th spell slots are fewer in number, as pointed out in Mindwin's interesting (although off-topic) question. Given that apparently the designers though that higher level spells should be cast only sparingly compared to 1st-5th level spells, this is my reason for being wary of allowing my monks to cast such high level spells potentially multiple times a day due to ki replenishing on a short rest.