I think that would work tho - the "new" Tchalla can be presented slightly different from Chadwicks, and create tension/conflict with our cast who don't know this version of him
he could come from a universe with the same events as ours except Wakanda got flooded by Namor (or another catastrophe) ... so while our universe lost Tchalla, in that universe Tchalla lost everyone else
it presents a cool duality for storytelling and how his experiences changes his approach
if they wanna bring Killmonger back, it could be cool to see how the new Tchalla may be more hardened than even our Killmonger, who might be softened after the events of BP1 and being bought back somehow
"new" Tchalla wouldn't even need to run Wakanda at first ... they could go the king of the dead route, or him just being the avatar of Bast; have Shuri or whoever run the country, and his arc could culminate with him "re-earning" his title as King and the confidence of the supporting cast who lost their version of Tchalla
edit: this could also address a lot of the complaints of Tchalla from the 1st film; the new variant could be distinguished from Chadwick's version, if he didn't have anyone to rely on for a while after he lost them, he would be more self-sufficient and wouldn't need Shuri to be his Q, and would be more prone to make harsher/militant decisions at first (tho they would prolly have to write him back to being more centered eventually)
also, if the heart shaped herb is truly all destroyed, he could just bring his own stash, which could explain why the OG cast have to accept him as BP because he is the only one that still has the herb, even if they didn't trust him or know him at first, or feel loyalty to their Tchalla, etc