The safest bet as to the identity of the new Black Panther is Shuri, T'Challa's genius younger sister. In the comic books, Shuri takes on the mantle of the Black Panther at one point while T'Challa recovers from serious injuries. Her position as the surviving heir of the Wakandan royal family, as well as her experience both in battle and in her science lab, would make her the obvious choice.
As shown in the first movie, any member of the royal bloodline from any of Wakanda's tribes can challenge for the throne (and the title of Black Panther), but the primary royal house — of which Shuri is the current heir — usually ascends the throne with relatively little conflict. It's unclear if those rules can be changed, however, which could open the door for other, non-royal successors.
A few shots in the "Wakanda Forever" trailer seem to show Shuri taking on a leadership role. However, she's still a scientist first and foremost — might she decide to turn down the Black Panther mantle and continue working behind the scenes instead?