Oh dear.
This point is where things could diverge. In one approach, we explain Bob's ability to conjure whirlwinds because he once conquered a region of the elemental plane of air, slew a mighty elemental prince of storms, and claimed his powers. In the other approach, we tell you that Bob is a controller, and because of that he has abilities that allow him to move people around in a fight.
Do. Both.
The adventure should almost certainly tell the DM that Bob can control winds because of that elemental thing. But it should
also tell the DM that Bob is a Controller.
It's like the selection of a PC's class - the player probably chose Fighter because he wants his character to be good at hitting things, and that's the appropriate class for that.
(And yeah, to an extent that's circular logic. Did the player choose Fighter because he fancied playing that class, and the "hit things" come after, or did he want to "hit things" and so chose Fighter? It doesn't actually matter - the bottom line is that the player has a Fighter
and he's good at "hitting things"... and his character sheet notes both.)
I believe that we can use both approaches, as long as we're mindful of how and why we're doing it. An entry in a book like the Monster Manual might be driven entirely by the first approach. The entry frames everything in terms of story and the immersive elements of the world of D&D. Monsters don't have roles, they have backstories and cultures.
Again, why can't they have both? The 4e MM did a really good job in presenting several different types of orcs/goblins/drow/whatever, each with different roles to reflect their place in their appropriate societies. The 4e MM went wrong in that it tended to skimp on background details, but that doesn't mean you
drop the mechanical expression of the roles, it means you
add the 'missing' background stuff.
After all, an Ogre is not going to be any less a Brute just because the MM doesn't list him as such.
For maximum utility, include both monster backstory
and the monster roles.
On the other hand, our encounter-building guidelines should speak to our more technical-minded DMs. We give crystal clear advice on how to balance encounters.
Fair enough...
We give you a list of every creature and tell you what it's best at.
Ah I see.
Put it in the Monster Manual, in the entries for each monster. That way, when you add more monsters, you don't have to print revised tables, and end up with wasted pages in the DMG.
The story DM rolls on random encounter charts or just picks the creature that feels right.
Or, and this is a radical thought, you could provide
storytelling guidance to such DMs. Take the time to talk to them about pacing, and character motivations, and all the rest. Explain to them different types of encounters (speedbumps, overwhelming encounters, etc), how they each fit into the story and how each can be used to a different effect.
The 4e DMG, to its credit, at least
tried to do this. Sadly, it largely failed (IMO). But here's a suggestion: Ari Marmell has done significant work for you in the past. He also has significant credits with White Wolf's Storyteller/ing systems. Why not leverage that expertise and have him write some guidelines for you?
Do you start with a list of monsters by level and role, or do you flip through a book looking for creatures that are greedy and foolish enough to strike an alliance with a cleric of Cyric? When the characters head to the Amedio jungle, is your first impulse to sort creatures by their typical climate and geographic territory, or would you happily reskin a yeti into a jungle brute if you liked the yeti's mechanics? If we're doing our job right, it doesn't matter which approach you prefer. The game supports both without making one or the other feel wrong.
I agree 100% with the principle of what you're trying to achieve. I'm just not 100% sure you're going about it the best way.