It is an opportunity, something out of the ordinary is happening in Shadowlands, so for a little while the usaul order of things is going to be disrupted, no one can usually comeback from teh dead, but the living can't enter the realm of the dead either. So the shadowlands expansion can provide a once only ever opportunity to return some major characters and even re-populate some heavily depleted peoples.
Taking it further it could accelerate the recovery of the Azertoh nations and peoples - Yes this is not usual, but it is an opportunity they could take. So .. who would you bring?
Kael'thas?
Staghelm?Quel'thalas elves slaughtered in WC3?
Arthas?
Ysera?
Neltharion?
Malygos?
Krasus?
Night elves slaughtered in WC3 and WoT?
Farondis highborne?
Lordaeron humans?
Gnomes?
Arathor humans?
Amani Trolls?
Obviously I chose a lot of elven peoples, becasue since WC3, they've been the main races to suffer genocide and need re-populating. I'm sure you can think of loads more.