I’m referencing the “spirits” because that’s Blizzard’s preferred terminology. What we’re now referring to as the Shadowlands has, for the longest, been the Spirit World. WoW’s afterlife. The place where souls go when they die.
Where else would Cairne Bloodhoof’s soul be coming from, if not the spirit world/afterlife/Shadowlands? Where else would Vol’jin’s soul be? Rastakhans? Where else would “the Other Side” that Bwosamdi sends souls to be? Where else would his boss be?
If the that logic is that shamanism/voodoo has never been linked explicitly with the term “Shadowlands” before now, then the same goes for Bolvar, Sylvanas. Sure, they raise the dead, but they’ve never been mentioned as tied to the Shadowlands or afterlife either. They deal pretty much exclusively with the unnatural state of undeath on the material plane. If anything, they seemed to prevent souls from going to the Spirit World/Afterlife/Shadowlands. It wasn’t until the new expansion previews that they revealed they were actually associated with it in the form of Maldraxxas.
As for who/what should be there? Well if I’m being asked to make a rough pitch…
All the honored dead, many spirits, and other entities regularly dealt with by the shamanistic/voodoo traditions of Azeroth should be there. Baine should be there. Vol’jin should be there. Sen’jin should be there. Bwonsamdi, Shadra, Rezan, and Hakkar and other Loa should be in the Shadowlands. Thrall’s mother should definitely be there, not kicking around the Scourge.
Aesthetically, buildings that resemble great stone/log house sanctums and mosleums, bonfires, totems (in the general anthropological sense, not specifically Pacific Northwest style), monoliths, feast halls, burial mounds. Shrines/altars that the dead visit in order to receive their offerings and contact from the living.
All surrounded by wilderness -forests, plains, swamps, mountains- in shades of red, green and gold. Inhabited by souls of not just Tauren/Orc/Trolls, but also Wildhammer Dwarves, Tuskar, Furblogs, and all the other races of Azeroth with strong shamanistic traditions. These also tend to be warrior cultures, so the souls that roam can have a similar martial bent. Maybe some are engaging in endless hunts/skirmishes and the player characters are just more sport.
The overarching conflict is obvious. The maw is eating up souls of those honored dead that should be living out their intended afterlives. An interesting dynamic could be, a divide between the warrior champions- who seek immediate and decisive action, but are held back by inter-tribal rivalries that held them back in life, and the honorable elders- who in their wisdom and age are now looking forward to an afterlife of peace and cooperation, but are dragging their heels as they engage in endless council sessions.