The Multi-Role campaign

In WoW, there are basically three types of greater quest chains/campaigns.

• The first (and oldest) is what I call "location quest chain" - any given region has a wide array of quests that oftentimes require the player to start from scratch with a quest, go through all of them in order, and ultimately culminates in a great event that finally rolls up the questing zone (and, since WotLK also potentially affecting the questing zone). The battle for Wrathgate would be a relatively early example.
• The second would be the "overall story campain", where the story stretches over multiple zones, possibly the entire expansion, where one (or the) challenge of the expansion is set up. The regularly recurring Lich king in WotLK would be the first example of this type.
• ·The final type is the Order Hall campaign, or "class campaign".

The idea of Multi-Role campaigns is to add another element of story-driven immersion to the game, and possibly (not necessarily) to replace the class campaigns. The class campaigns in Legion added an element to the game that could be described as "SWTOR class campaign light".

The weakness of SWTOR's class campaigns was that while all of them had a notable and serious impact on the story (the situation as it turns out towards the end of the main game is the result of the accumulated events that took place during each of the 8 class campaigns), there was next to no crossover content between the different classes, in a way it was as if each of the campaigns (with its protagonists and antagonists) was taking place in its separate bubble: for example, awesome and powerful NPCs from one class campaign didn't even get a passing mention in the other campaigns apart from very very few instances.

Legion's Order Hall campaigns have a similar weakness - while the central NPCs are partially given more screentime (Velen, Odyn, Khadgar...) the campaigns as such are mostly isolated from one another, occasional cross sections (a Paladin hero defending the Netherlight temple, for example) notwithstanding. Which is a shame - I only got wind of one of the most interesting campaigns, the Death Knight's effort to reassemble the Four Horsemen, because I checked wowwiki and youtube.

So what's the Multi-Role campaign supposed to be?

Simple: a quest chain (or rather: a quest web that consists of quest chains) where the player is rewarded for playing through a multitude of class campaigns by getting additional elements of the story disclosed in one campaign the further he moves down in others. It's supposed to work as follows:

• The Multi-Role campaign should NOT be mandatory. Artifact quests, legendaries etc. should not depend on it. The rewards can be quite useful, but they should be either inconsequential (immersion rewards), non-essential (like wealth) or be relatively easy to replace by doing other content (moderately powerful items).
• The individual Multi-Role campaign quest chains do NOT consist of individual quest chains for each individual class in the game, but of quest chains for roles instead (with each role open to several specific classes, see below for a possible setup). This is done for three reasons: the first is that players shouldn't be forced to play each of the 12 classes in the game in order to complete the Multi-Role campaign, the second is that players shouldn't be strongarmed into playing classes they don't like to max level - a player who is a bit flexible should find at least one class he likes to progress down one specific role quest chain. Ideas for quest chain class combinations are further down. And the third is: with each additional role, the intersections between the different roles would grow exponentially. So keeping it relatively low also keeps the effort manageable.
• Multi-Role quest chains work as such:
• The player progresses down the path of a certain role up until a point where no more solo quests are available.
•  If the player progressed down on the path of other role campaigns, he unlocks cooperation quests. Cooperation quests have multiple characters run into each other and take on new challenges together.
• During such a cooperation quest, the player basically has his own alt(s) taking on the roles of NPCs. While the player has to beat the challenge with his current character as usual, his alts pull off a textbook version of their own questrun by default. Of course, once the player has to do the quest with his other character(s), the roles are inverted: now his main character is doing a flawless questrun (even if he didn’t in reality) while his alt has to beat the quest. The idea behind this is to give the player the impression that his character isn't just acting in a vacuum, but that all his characters exist in the same world, that they are aware of each other, and that they're working towards the same goal.
• The other characters look exactly the way they do at this particular moment. I.e. even if a player has beaten a quest with one character and only does it with the other character weeks later, the player looks like he does at that moment and not the way he did back then (think of the time manipulation bronze dragonflight quest in Dragonblight).
• In order to progress further on a given path, the player has to beat the cooperation quest from all possible perspectives with all characters. However, a player does NOT have to complete all quests on a given tier in order to advance – all he needs to unlock a quest on a specific path is to have progressed far enough with all characters involved. For example, this means that in order to take on a three character-cooperation quest, the player must have completed all 3 available combinations of two character-combination quests before, but not those including the characters who aren’t involved.
• The cooperation quests include more and more characters (at least as long as it’s feasible), until the singular final quest where all characters are taking part (provided the player has enough characters for that).
•  Since each class can potentially fulfill several roles (see below), it's possible to stick to only a few classes. However, one character can't fulfill two roles simultaneously, meaning that in order to complete the Multi-Role campaign, one needs a separate character for each campaign (i.e. six individual characters in the role setup presented below).

Possible roles, their class combinations and the campaign setup may look like this:

• Darksider (Warlock, Demon Hunter, Death Knight, Priest): The darksider is a character who is in touch with the powers of darkness, whether they’re Unead, from the Twisting Nether or the Void.
• Slayer (Warrior, Rogue, Hunter, Paladin): The slayer is a powerful fighter who seeks out the strongest foes, whether they’re monsters in their lairs or Warriors on the battlefield.
• Warlord (Warrior, Paladin, Death Knight, Monk): A gifted general, the warlord leads the loyal armies under his command into battle.
•  Sage (Druid, Priest, Monk, Shaman): A wise, spiritual character who serves powers greater than himself, be they the Light, the gods, the elements – or his people.
• Stalker (Druid, Hunter, Rogue, Mage): Shady and swift, the stalker is a stealthy character who uses his skills and powers to get to his target unseen and disappear without a trace.
•  Wizard (Shaman, Mage, Warlock, Demon Hunter): A powerful caster who commands the elements thanks to the secret knowledge he possesses. 
• There are six beginner campaigns (each from a single perspective), 15 two character-cooperation campaigns (each from two perspectives), 20 three character-cooperation campaigns  (each from three perspectives), 15 four character-cooperation campaigns (each from four perspectives) and one final campaign that rolls up the campaign (played from all six perspectives). 

Or some similar combination thereof. Of course, should the "class variant" concept be implemented (check the Class variant blogpost), this looks radically different. A concrete example of how a multi-role campaign would look like under the given circumstances (warlord campaign):

1. Getting started: The player, playing a Warrior character, triggers the multirole campaign. He’s asked which role he wants to play (in the case of the Warrior: warlord or slayer) and decides that he wants to try out the warlord campaign.
2. The beginner campaign: The first quest chain is about the building of an army. The kind of army the character builds depends on his class/faction. A Warrior builds a standard Alliance/Horde army, the Paladin, Death Knight and Monk classes build a class army (Silver Hand/Shado-Pan/Ebon Blade). The second quest chain is about doing battles. After he has finished the starting campaign, the warlord campaign is put on hold until the next step is triggered.
3. The first 2 character-cooperation campaign: After the player has finished at least one other beginner campaign, he has unlocked his first cooperation campaign. The nature of the cooperation campaign depends on which other campaign he has progressed with (for example, the stalker would unlock a siege campaign where the warlord character besieges a castle while the stalker enters the castle, murders the guards, opens the gates and assassinates the leader of the defenders). After he has finished the first cooperation campaign, both campaigns are put on hold again until the next step is triggered.
4. The second 2 character-cooperation campaign: After the player has finished a third beginner campaign with yet another character, he has unlocked his second cooperation campaign. The nature of the cooperation campaign again depends on which other campaign he has progressed with (for example, the slayer would unlock a battle campaign where the warlord character leads his armies into battle while the slayer runs into the fray and kills the general of the opposing army). After that the campaigns are put on hold again until the next step is triggered.
5. The third 3 character-cooperation campaign: After the player has finished both 2 character-cooperation campaigns with both of the other characters (in this case slayer and stalker) AND finished THEIR 2 character-cooperation campaign from both perspectives (all in all having played each of the three 2 character cooperation missions twice), he has unlocked his first 3 character-cooperation campaign. The nature of the cooperation campaign once again depends on the other characters (this time, the warlord makes good use of both his champion and his assassin to rout yet another army/take another castle/whatever).
6. And so on, until the player has finished all available campaigns with all six characters and is finally able to take on the last campaign with all of his characters (this quest should be really tough, but not to such an extent that only hardcore raiders are able to beat it).

Multi-role campaign diagram with cooperation tiers.