I think Blizzard missed a way to connect their gaming worlds together. In the Maw intro, Manduin opines that there are beings from Azeroth, Draenor, and numerous other worlds pouring into the Maw. However, all we really see though are Azerothians, from what I remember. I thought it would’ve been groovy to some across Maw/Shadowlands characters who were from other Blizzard games such as Diablo, Starcraft, etc. There’s one daily in Bastion where you have to determine the final fate of three different characters. Throwing in a Zerg, for example, and determining its fate would’ve been neat. Also in Bastion there’s the quest where you play as a dude named Ben and watch as he ends up getting tossed into the Maw. It could’ve been interesting to have different scenarios set in the worlds of Diablo, Starcraft, or even Overwatch.
Sadly those other game universes don’t coexist with the Warcraft universe.
May I politely ask if you’re sure? I though I read that Blizzard said they were in different parts of the same universe (though I admit I could be wrong or the information faulty).
Other worlds = Other planets within the Warcraft universe. As for why we only see Azerothian spirits, Blizzard explained this. When we look at a spirit in the Shadowlands, we see, at least for the most part, what that spirit looked like in life, because we can make that visual reference. All other spirits appear as formless shapes.
A creature from another world, that has no idea what an Orc is, would see spirits of their own people, but would see Orc spirits as a formless shape.
If they said that at some point, I would expect that Heroes of the Storm and the “Nexus” sorta retconned that possibility.
Ya, I don’t know. I remember reading that they said that Starcraft and Warcraft existed in the same universe but there was no context to it.
You don’t have to ruin the other games by bringing them into shadowlands
In WoW, the only alternate universes are alternate timelines of the main universe. There are no known independent universes. With the current lore, any canon crossovers are impossible.
why is the lore so bad man
I don’t recall any quotes about that…
There were the easter eggs in Warcraft 3 and Starcraft 2 about Space Marine Orcs and Hydralisks in WC3 and Tauren Marines in SC2 but they are more non-canon jokes than actual appearances. (And of course the little Diablo you can find on that one WoL mission with the rising lava.)
I do think they missed out on using the Sargerite Keystone from Legion to explore other worlds and leave Azeroth behind to recover from all the apocalyptic events and have minor cross-over encounters.
A true Demon invasion event with a cross-over Diablo raid similar to the Nier Automata factory raid in FFXIV would be pretty cool. Like a powerful Eredar of the remnants of the Burning Legion is seeking revenge, and having gathered an army of demons and built a soul-forge to focus their essences and restore them should they die is invading Azeroth and in the climactic encounter in their desperation to survive/kill you/escape they rip open a dimensional portal that lets in a part of Hell from Diablo and you have to battle through Hell’s minions and defeat a bunch of minor well-known demons from the Diablo verse and the final encounter is sealing the breach. With perhaps as the breach is sealed you hear the voice of Mephisto (That voice still freaks me out.) asking “What is this?” Now you have to wonder if you saved Azeroth or only let a far more sinister and evil world learn of its existence.
But that would be the extent of a cross-over encounter. A Dimensional tear, minor characters and references… Perhaps the hint of the more major lore characters from those universes, but nothing egregious like: Boom the UED pops in a Battlecruiser and joins the Alliance, now the Horde has to ally with Sarah Kerrigan or be destroyed. 
I definitely would NOT want to have quests from Jim Raynor, or Tyrael, or Tracer but little things like seeing a random Zergling on a strange world and wondering what type of Felhunter mutation it was… little things like that spread throughout the WoW-scape that hint of a much larger and connected universe.
An inability to understand the lore doesn’t mean the lore is bad.
that lore is horrible
an inability to make one universe matter doesn’t mean their afterlives should be linked