I think I have a way to bring Erik back (sort of) without cheapening anything.
Consider this:
We open the movie in Wakanda, where the council is discussing Shuri's attempts to recreate the HSH from the minute traces of DNA left when the garden burned.
A report comes in that the dead walk in the lands of the Merchant Tribe (picked because we didn't see enough of them last time).
T'challa, along with the DM and his new Hatut Zeraze commandos (led by a Border Tribe expat named Kevin Cole?) go to investigate, and are confronted by the undead forces of Baron Macabre, who is accompanied by a masked bodyguard. While the DM and HT take on the zombies (with limited success.. they simply aren't trained for this), T'Challa goes after Macabre himself.
The bodyguard intervenes, and when T'Challa manages to crack his mask, he's stunned to see the face of his late cousin. The shock allows him Macabre to affect him with some sort of hallucinogenic gas, and make his escape.
Back in the Golden City, T'Challa recovers, but that night, he undergoes a vision quest to reach the ancestral plane, where he meets his ancestors (as well as Zuri and a few others) who teach him the true power of the Panther God. Along the way, one spirit (who looks like Sterling K. Brown) tells him that there is one who has not found peace in the bosom of the Panther God.
He awakens, and summons his most trusted warriors... but less than the first time. Some, like M'baku, wonder if he is not fully recovered, but T'Challa merely says that Bast has shown him the way to victory.
They return to the region under Macabre's control. T'Challa stands before the army of undead, not even armored. The Baron scornfully asks "how do you think you can possibly win over those who cannot die? Who do you think you are?"
"I am T'challa. Son of T'chaka. King of all Wakandans. Both living... and dead."
And the ghosts of Panthers past appear in formation behind him, including young T'Chaka, and anyone else they feel like casting.
The battle is joined. Ghosts vs Zombies. T'Challa makes a beeline for Macabre, who sends zombie!Killmonger to intercept... when a new spirit rises to confront it.
"Heya cuz... 'sup."
(Yes, Jordan would play double duty here. I doubt he'd mind)
T'Challa defeats Macabre, the undead army and the spirits of the dead depart. The last to leave is T'Chaka, who assumes his elder form, and shows T'Challa an altar in the highlands where the wild HSH grows. The line of the Panthers will continue. Wakanda... is forever.
Mid-credits: Killmonger's body is lying in a crypt, now one of the honored Panthers of the past. A shadowy figure stands over it, pouring some sort of elixir into its mouth... and his eyes open.
"What? Why have you brought me back?"
A dreadlocked woman, wearing worn Dora Milaje armor, steps into the light. "Consider it an act... of Malice."