This is part 3 of a probable 4-part series on designing spellcasting features. Part 1 can be found here and Part 2 can be found here.

Let's talk about partial casters!

By this I mean half-casters (paladins, rangers) and third-casters (Eldritch Knights, Arcane Tricksters). We can pull in some good homebrew classes and subs as well. Prominent half-caster classes include my troubadour and bloodrager, and KibblesTasty's inventor, spellblade, and summoner. Noteworthy third-casters include my Thornfinger rogue (1/3 druid), Kibbles' Crusader fighter (1/3 cleric), and my Trickster swashbuckler (1/3 illusionist). (While my 3/4-caster necromancer is technically a part caster, it functions much more like a full caster than a part).

These archetypes are often mathematically a little weak-looking; they get spellcasting as a major part of their power budget, but without higher-level spells or as many slots, they can't keep up with a full-caster on the basis of spellcasting output. Half-casters cap out at 5th-level spells, and in the all-important level 5-10 range of the game, only have 2nd- and 3rd-level spells and 6-10 slots. Third-casters are even more constrained, capping at 4th-level spells and only having access to 2nd-level slots in tier 2.

So partial casters can't rely on their spells for sheer power output; they neither have the scaling nor the slots to make that work. Instead, part-casters use their spells for important utility effects or, in combat, to augment the other parts of their toolkits.

Note that while I referenced the spellblade class above, I won't talk about it too much; I'll be saving it for part 4, which will specifically go into detail about gishes.

Stronger Spells

The defining feature of a part-caster's spells is that they're, frankly, not that strong. This can vary; a Charisma-focused troubadour will have a good save DC, and punches above its weight thanks to a heavy emphasis on buff and control spells (rather than damage spells, where lack of higher-level options hurts more). So if your version for a part-caster subclass involves heavily relying on spellcasting, you'll want to look long and hard at the straightforward power buffs.

But! You need to follow one important rule: no matter how cool your part-casters gets, it still needs to be a worse spellcaster than a full-caster would be. For example, Kibbles' Crusader fighter gets a 10th-level feature named Strength of Faith, which lets it add half its Strength or Dexterity modifier to its spell attacks and save DCs. While it's a great pun and feels delightful in play, it makes it relatively easy to get a higher save DC than the party cleric has (especially using the fighter's extra ASIs). This can produces feel-bad moments at the table and can undermine the fabric of your world (because does it really make sense for the fighter's spells to be harder to resist than the cleric's?).

Numbers Go Up

These features are pretty tricky to give to part-casters. It's too easy for them to cleanly outstrip a full-caster's equivalent spells. While sometimes a straightforward number buff can work, I'd suggest you focus your attention on other feature types.

More Reliable

Reliability features are probably the biggest category to look at. Eldritch Knights can force enemies to save with disadvantage against their spells; while I personally hate forced-disadvantage features, it's the sort of thing that's made for third-casters, specifically. (Literally: both of WotC's third-casters get a forced-disadvantage feature.) Damage rerolls work well but may still not be enough to get damage spells to make sense. Retargets and other creative ways to save a failed spell can also be great.

More Upcasting

I suspect WotC's official position here is that giving upcasting features to part-casters steps too much on a full-caster's toes; they've been very shy about this style of feature generally, and it would make it easy for a part-caster to look more like a full-caster's power. I like them, though. I think it's a great way to make spells more attractive, and as long as you don't grant an upcasting feature in tier 1, your part-caster will still not have as much juice to their spells as a full caster does.

More Spells

These are fine features for partial casters, but usually you'll have some form built-in. If you're writing a third-caster, you should already have a vision for what spells they're getting access to. Every half-caster I can think of will have a bonus spell list baked in as part of its subclass template.

More Spell Slots

This one's...tricky. Because part-casters are tightly constrained by their lack of spell slots, giving them more is really cool, but also can erode their identity. I don't think I would ever do this on a third-caster; playing a third-caster with bonus slots just raises the question of "why are you playing a third-caster at all?" On a half-caster, this can be a great way to bridge the gap towards a full-caster archetype. For example, my Spellsinger troubadour gets a ton of extra low-level slots, letting it spam out weaker spells with abandon. You won't want to go overboard, but you can get good use here.

Spells as Triggers

I said last time that this is key to making cool features for full casters; it's maybe even better here. Trigger effects give you a lot more room to incentivize the fantasy you're building. For example, my Knight-Evoker subclass (a rebuild of Eldritch Knight, sort of) gets a damage bonus to its next attack after it casts a spell. It's a simple trigger, but it encourages weaving attacks and spells together (and quietly, you can budget it as an increase to spell power that just happens to occur on attacks).

Maybe the most straightforward trigger is to build a resource. My Inquisitor swashbuckler generates panache when it casts spells; various Abomination subclasses generate frenzy when they cast spells. The downside to this is that you've got a subclass now with one resource (whatever its basic resource is) on top of spellcasting; that's fairly high complexity, but as long as you budget for it, it's workable.

Casting Time

It's really easy to write a part-caster a feature for "When you cast a spell, make one attack as a bonus action." Congratulations, you've made a gish! We'll talk about 'em more next time.

If you limit a feature like this to just leveled spells, then you might be able to balance the gishing impulse with the archetype's other needs. If it works with cantrips, then no, really, you've just written a gish. Cantrip-plus-attack is pretty much all it'll ever do again.

Conditions

Condition interactions are interesting on part-casters. Their save DCs probably aren't as good, so they can't rely on using spells to inflict conditions. But they might be able to use the rest of their kit. For example, it's straightforward to give a paladin or a fighter features to knock enemies prone; you can then give their spells a boost targeting prone enemies. You could imagine writing a ranger sub that gets lots of entangling-vine features to inflict restrained, and then triggers bonuses based on it.

Concentration

Concentration boosters are often not interesting on partial casters; their concentration slot just isn't as important as on full casters. What can be nice here is to boost their ability to succeed on concentration saves, since part-casters are much more likely to be melee combatants. Advantage can be a lot; I have used "You succeed on the first concentration save you make for any given spell" a few times and quite like it.

Loaner Spells

Loaner spells are very hard to justify on partial casters, who already will struggle against a limited number of spell slots and spell levels.

Vancian Spells

Hot take: Artificer should have just been a Vancian caster.

My commentary on Vancian spells for full casters was "they can be nice, especially if they consume your plentiful spell slots." For part-casters, the opposite is true — Vancian spells can be a way to sneakily buff your spellcasting capability, especially if they don't cost your limited slots. Especially for third-casters, who are really constrained.

Next Time

We'll finally wrap this up! For part 4, we'll focus on that most dreaded of spellcasting archetypes: THE GISH.