With the design of 5th edition still underway, I wanted to rant a little about the ability (or rather the current inability) to add PC classes to monsters. Being able to make an orc into an orc fighter is pretty vital to my enjoyment of the game, mostly because making a classed creature equates with the DM’s ability to make NPC opponents.
This is a topic with some baggage, which needs to be discussed and acknowledged: past attempts have led to preconceived assumptions of what mixing classes and monsters means.
What Has Come Before
Almost no monsters had classes in the first couple editions. You had monsters and you had PCs and the rules were sketchy when you tried to make an encounter with say human fighters or an evil necromancer. You could add classes to some humanoid monsters, and creatures like orcs were often given their distribution of classes and levels. This was always a little awkward as it was presented randomly, so you could create an orc encampment as a random encounter with a chieftain and his bodyguards and shaman. But adding classes to monsters was not quick and the rules for doing so (and awarding xp) were a little muddled. And, like all encounters prior to 3e, determining the challenge of the encounter was nebulous.
3e unified monsters and PC rules: the math was the same and all the numbers worked together. Classes became a common way of customizing and tweaking monsters. If you needed a monster to be a slightly higher CR then you could add a couple fighter levels. This was most common with the humanoid races, who needed class levels to advance. 3e also allowed monsters to be modified by adding Hit Dice as well as by templates. Templated were fun and allowed the DM to easily make a creature celestial or vampiric or a half-dragon and removed the weirdness of earlier editions where the Hit Dice and powers of a creature would radically change when it became a vampire or werewolf.
The downside to the 3e version, was the math. Monsters were as complicated as PCs and there was a lot of finicky bits that existed to make some more arbitrary aspects of the design work. Building a high or even mid level orc opponent was a lot of work: there were feats to track, skill points to add, stat bonuses to consider, and generally a whole lot of mathematics. It was a long, slow process. Templates, while ostensibly simple, also often required a fair amount of rebuilding and math, being very time intensive. Changing something as simple as the size of the monster meant recalculating its ability scores, factoring in skill bonuses or penalties related to size, grapple bonuses, reach, and the like.
4e dropped the idea that PCs and monsters had to share math and rules, and also dropped adding class levels to monsters. Instead of adding class levels to increase or decrease the potency of a monster, 4e allowed DMs to rapidly increase or decrease the level of a monster. 4e also kept templates, which were used to make monsters elite or a solo. There were also class templates, which adding a dash of class flavour to a monster while also making them an elite or solo. Late in 4e they also added monster themes, which were pretty much powers you could add to a monster to give it a particular tone or make it fit a particular area. These were great although underutilized, only appearing in a couple products.
The disadvantage to the 4e system was that making something like an orc fighter meant you either added a template (giving the monster a larger role in the combat) or you had to design an entire monster from scratch, which could be just as math-heavy and time intensive as adding classes to a 3e monster. The needs of a good elite or solo were also different than the requirements of a classed monster. As bad as many early elites and solos were, templates were worse and did a poor job upgrading monsters to elite status. And because classed monsters were at least elites, there was no easy way to pit the party against a rival group of adventurers... excluding designing a party of unique monsters from scratch.
The 4e books were also bad at teaching DMs how to design monsters. It explained the math but most but much of the subtleties were lost or unexplained. As monster powers were often designed very differently from PC powers, it was less easy to just port over powers.
All of this made NPCs difficult to design.
There are a few reasons why adding classes to a monster is beneficial. First, it allows for much greater available resources. Instead of being limited just by the options of the monster books, the DM has all the resources of player books at their disposal. DMs that buy player-centric books get a use for that content, an excuse to actually use their purchases.
This can also help mitigate PC power creep. Instead of having to rely on continued new monster books with upgraded options, DMs can use PC options from newer sources that might be of a higher power level, turning power creep against the players. Similarly, optimized PCs can face opponents of equal challenge and potency.
There’s also an advantage to known commodities. The same classes have existed for forty years because they’re evocative and descriptive. Creating a new variant of the bugbear such as a “bugbear cuthroat” or bugbear footpad” is simply not as elegant as just having a “bugbear assassin” or “bugbear rogue”. Classes carry narrative weight.
Similarly, monster power X blends into monster power Y. Unless the players have read the Monster Manual or the DM stops to read power aloud it has limited impact. It’s just mechanics and it’s difficult to put flavour into monster stat blocks without killing space. In contrast, fireball or channel divinity or bull rush are recognisable, they are known by their names which are evocative enough to need no other description, but also player books have the advantage of being able to pair flavour with mechanics without having to fit everything into a single succinct statblock.
Additionally, sometimes you don’t want a monster. Fantasy fiction has a history of evil wizards, corrupt rulers, and other human opponents. 4e struggled with this, having each be a unique monster rather than customizing existing monsters. There were dozens of different orcs all of varying levels in addition to XXX humans and YYY elves. Short of having pages of human or generic humanoid monsters in the Monster Manual, NPCs will always, always have to be constructed. Relying on pre-built monsters drastically limits options for human-centric games, such as urban campaigns or worlds without a myriad of intelligent humanoids.
While it’s possible to reflavour monsters as humans, this isn’t always satisfying. Why would an NPC warrior - especially one like a common town guard - know a maneuver impossible to the fighter? Why would an apprentice wizard be able to cast a spell unknowable to a high elf wizard?
This always reminds me of RPG video games (typically J-RPGs) where you sometimes fight a character before they join your party and they kick the entire party’s butt and use moves they somehow forget once they become a party member.
(Aside: This is one way relying on pre-built adventures and internal playtesting might fail the game. The designers are comfortable enough with the rules - and skilled enough designers - that any difficulties making NPC opponents are unnoticed. And initially, DMs will default to designing adventures around the content available. Absences tend to be noticed in longer lasting campaigns, when the story dictates something that might not exist.)
The Option of Complexity
The catch with adding classes to monsters is that it must be simple. 3e and 4e both failed with this endeavour. And the challenge provided must be more accurate, something that was hard to guage in 1e and 2e. As such, there really needs to be a few different ways of customizing monsters.
We need the simple levelling up and down of 4e, so DMs can quickly and effortlessly tweak the difficulty of a monster. The simple customization of monster themes is another simple way of tweaking monsters that should be included and emphasised. Monster themes should replace much of the design of templates from 3e (and 4e). Celestial, fiendish, and draconic creatures could be easily handled by dropping powers onto a monster, possibly paired with an increase in level if needed. Some class options could be handled by themes, giving orc, humans, or even mind flayers a dash of fighter or wizard.
For DMs who want more granular control and have the free time, there could be more complicated templates that rebuild monsters and methods of adding classes to monsters. This might (read: should) be closer to the multiclassing rules. Monsters don’t need the full range of class options and just need to be recognisable as that class through the use of familiar powers and class features.
Lastly - given monsters and PCs have completely different math - we need complete monster building rules. This is because NPCs in combat are designed like monsters, and sometimes you need a level-appropriate evil king, a necromancer, or something as bland as a town guardsmen or innkeeper. And, really, I’d like a Monster Manual filled with chimera, hydra, and dragons and not dozens of assorted NPCs across multiple level bands. An important note regarding NPC design just including the math is not enough. We need advice on what powers to include, what to avoid, what powers work best at higher or lower levels, how PC powers differ, and the like. Little things like “don’t give incorporeal monsters attacks that weaken” or the possible effects of giving a monster multiple Martial Damage Dice when paired with a lucky crit.
Heck, a regular bi-monthly article on monster design wouldn't be a bad idea.
Obviously this all needs to come later, when the rules are in a more solid state and the monster math has been finalized, but it’s worth thinking about now before the game gets too far along.