The whole point of a sequel is to switch things up. A bad sequel relies on what works and doesn't take necessary risks. I know not everybody likes it here, but part of what makes The Dark Knight great is that Joker was an intellectual challenge, not a physical one.
This isn't about making a copy of that movie, but understanding why that one works. Batman in that film is never really in any danger, but we know that he is losing because he's either failing to save people from themselves or the Joker, or his own moral compass is being used against him. It's compelling to watch.
What made Killmonger a great villain was that he had a point and that changed T'Challa. Being able to beat him was the bonus. You don't want to repeat this however, so you do a villain that purely intellectual and the narrative tension is whether or not T'Challa will commit to his ideals.
At the end of the first film, T'Challa makes a very bold step to open Wakanda's borders and share their resources. It's s breach in centuries worth of tradition and isolationism. The sequel needs to address and I think the best way would be with an intellectual villain. Therefore his external conflict would be about whether he can keep Wakanda and the rest of the world from harm because of his decision.