If I mandated that full casters take 1 non-spellcasting class level for every 2 spellcaster levels, would they be balanced with 2/3 casters?

Question

I've been asking about how to balance player characters, and others have suggested using tier 3 classes only, or using only spellcasters who cast 6th-level spells.

This is not the place to contest those ideas, even if they happen to be the starting point for what happened next.

My players want to run a campaign (the Giantslayer Adventure Path) where they all play dwarves, and dwarves have a malus to charisma. This arguably cuts out the bard, the skald, the bloodrager, the summoner and the paladin unless they take the Tortured Crusader archetype.

Along with the ban on wizards, sorcerers, druids, clerics, shamans, oracles, arcanists and witches, the playable classes are greatly reduced.

What would happen, balance-wise, if I were to introduce full casters again, mandating one level of a non-spellcasting class every 2 spellcaster levels?

Specifically, I have a player who would like to play a druid focused on earth magic, which I think is a great concept, but under the no-full-casters rule this is just not possible.

I expect the reduced caster level to harm those multiclassed characters more than the actual lack of favored class bonuses.

Examples:
Would a hunter 17 be better or worse than a druid 12 / ranger 5?
Would a warpriest 17 be better or worse than a cleric 11 / fighter 6?

Note: I will also be using martial initiators from Path of War, who work best when multiclassing due to their peculiar mechanics. I already expect the answer to be different for them (also because a fighter 6 / stalker 11 can reach level 7 maneuvers).

Answer 1

The multiclass characters will be a lot worse

There are several reasons for this.

Caster level

Like you say, caster level for the multiclass casters will be lower, which will hurt them a lot. Casters who use offensive spells will be hit hardest due to Spell Resistance, but almost every spell scales off of caster level in some way, and that'll all be 2/3rds as high for the multiclass characters.

That said, there are feats and options to mitigate this, and as a GM you can take steps to mitigate it further.

Lack of class feature support for the playstyle

The main problem with playing a caster/mundane hybrid - either through multiclass or as a hybrid class - is that you're still bound to the action economy. Every round, you'll essentially have to pick if you're a bad fighter or a bad caster that round. The only real way to make effective use of both at once is to use buffs prior to combat (which severely limits the types of casters you can play.

Many of the hybrid classes solve (or at least attempt to solve) this issue by introducing class features that try to unify them, or at least help their primary aspect along. A Magus gets Spell Combat so he can use both spells and melee, while a warpriest gets Fervor to cast spells on himself.

Stunted secondary class features

Best illustrated by the druid, the class won't lose just it's spellcasting. Wild Shape and (unless you pair druid with ranger) Animal Companion will also be hit hard by the 2/3rds limitation. Those two especially scale badly because the resulting forms/companions will lack the attack bonuses and AC to properly fight.

Other caster classes aren't hit as bad because they don't tend to have as many secondary class features (and those class features don't scale as badly), but things like domains and familiars will still be less powerful.

Meanwhile, the hybrid classes that have those secondary class features will scale at full speed.

Smaller problems

Like you say, favored class bonuses will hurt the multiclass characters. If I were to implement this rule as a GM, I'd just give the impacted characters 2 favored classes (since I'm the one who's forcing them to multiclass).

Depending on the pairing, BAB, save and skill progression might suffer. Some actually get out ahead, like the two you mention, but any pair with a sorcerer and wizard will suffer in the BAB department.

Answer 2

Placing restrictions like yours will indeed bring the fullcasters down in power, but doing so is complicated, and forces multiclassing on your players, which you might not want. It also prevents characters from accessing their highest level spells (a strong reason to play full casters in the first place).

There is a better solution available in an alternative ruleset: the spheres of might and power (Spheres of Power; Spheres of Might)

This rulesystem does away with spellslots and changes magic into a more "game-ey" style, where mages have a selection of powers they can always use, but only 1 at a time, and an amount of "spellpoints" to either make the effects stronger, or to extend the effect allowing a second effect at the same time (or both). This style of magic makes mages feel more magic: They never run out of "spells" (only out of stronger version), while the spellpoints pool limits their overall power per day, and the "one or two at the same time" limits their short-term nova power.

The spheres of might provide a similar system for non-magic classes (in a manner akin to 'path of war'), which boosts the nova and overall power of the classes. Combined: these two bring magic and martial classes a lot closer together in terms of overall power, it makes mages useful from level 1, and it keeps martials useful up to level 20.