Since 5e doesn't have the same problem with skill scaling that 3.x did I might be able to use the increasing difficulty as a way to curtail use.
As is I'm looking at a basic mechanic of Words having reliable basic effects on par with a Cantrip and a Word Modifier that affects the DC of the check required to use more than 1 word at a time.
Words are broken up as Descriptors, Things, and Actions. You have a limited number of each known and level determines how many you can use at once. How they interact when you use more than one will require either specific rules in each Word description or general guidelines. I'm probably going to go with the former.
So if you know Big, Fire, Fast, and Strike, you can use any one with little difficulty that scale like Cantrips. Example: At 5th level You compose "I Strike the target" this is a level appropriate damage effect so it's 2d6 save for nothing. Alternatively, you could write "My fighter friend Jarn Strikes the target." In this case it's still just a 2d6 damage effect, save for none. This has no impact on the Action economy since the damage of Jarn's attack is based on your Cantrip damage and you used your Action to do it (though it does have some passive benefit of Jarn seeming like a multiattacking terror and you looking like a harmless nerd). You could also script "The ogre Strikes the Chandelier." This could have different effects based on intent. If intent is bang head on metal thing, then the ogre gets a save. If the goal is having the ogre knock the Chandelier down accidentally, then the Ogre doesn't get a save, it just hits the Chandelier and the DM would have to adjudicate the effect, no different than acid splashing it. 2d6 might be enough to bring the Chandelier down, but it might just snuff some of the lights creating shadows for the rogue to hide, etc.
As soon as you combine 2 or more Words your effect becomes save for 1/2 and has a Calligraphy DC (10+word modifiers which range from 1 to 5 depending on their effects).
By default you can complete a Testament with 1 Word as an action. Adding another Word is a bonus action but now invokes the DC. So say it's 13, You can also amp the DC by some amount I'll have to do maths to figure out (I'm thinking 3 for a Reaction or 5 for no Action). so you can reliably use 3, 4 or 5 as you go up in level. I'm thinking the damage stacks (so like multiple cantrips landing) and the damage die increases for every even word, so 2 words become d8s, 4 words become d10s.
So by level 20, if you can crack the 5 word DC (which can range between 15 and 35, but I think damage effects will default to higher DCs to make combining more than 2 prohibitive) you'll be hitting for 20d10 (if for some reason you can combine nothing but damage dealing words), but more commonly 12d8-16d10. Each time you use that effect, the total DC increases by 1/word, so in this case +5. This also means at level 4, when you'll have around 5 words total you might be able to pick 5 that work together and hit that DC, but the damage will be 5d10 since we're cantrip scaling.
Hmmm, that does create quite the spike at Tier breakers 5, 11, and 17. I think some clever design in the Word choice and level availability can keep that more manageable.
The real trick will be lasting effects. I'm thinking Lasting will extend a given effect by 1 round as a Cantrip (so you could use your action to turn the Bladewarding Warlock into a somewhat poopy Barbarian... is that too much?) and when mixed it creates a 1 minute duration. Maybe it can scale based on number of Words, so if you do a Lasting Fire the fire appears for 1 minute. Lasting Big Fire would be a 1 minute large area, while a Big Lasting Fire might last for 10 minutes.
Big just being a x10 multiplier for area or volume feels okay if effects like Fire are "1 ft diameter ball of flame."
If the Word Modifier increases by 1 for Each time it's used and reset after long rests, then I think that will be a good limit. Gotta see where Expertise comes in, Probably gonna hand it out so the math accounts for it right away rather than having a level of rogue break it all to heck.
You could buff an ally performing a feat of strength with "Jarn is Big." So Jarn counts as large until the end of your next turn.
You can just write "Fire" and there's a level appropriate cantrip effect. Fast might increase move speed normally, but when combined with others it might make much stronger effects. I think the basic vocabulary will comprise between 30 and 45 words, you don't know all of them, you know a portion and can forget/relearn every Calligrapher level like a Sorcerer.
The idea is that you're trying to elevate one of the two languages believed to be derivative of Truespeech, Celestial and Primordial, into Truespeech and coaxing the universe to "remember" the events as you describe in your testaments. I'm borrowing thematically from my Artist Class
, but I don't think I can plagiarize myself.
Subclasses are focused on how you record events. Runekeepers use Runes which empower elemental effects, Codex keep books of prophecy in riddles and code that reveal secret Divinations, and Illuminators embellish their Testaments with elaborate renderings that create or enhance illusory or Conjuration effects.