The Legend of Tarzan may not have blown the doors off the box office, but it put up respectable numbers for its opening weekend. It's also getting solid fan support on Rotten Tomatoes. If it can continue on this pace, it's certainly a possibility that the movie could do the necessary business to still become a success, and if that happens, a sequel is certainly in the cards. Since Edgar Rice Burroughs wrote over two dozen Tarzan books in his life, there are numerous places the character could go from here.
The first film took a number of bits and pieces from Tarzan lore to create its story and added a healthy dose of reality for good measure. While Edgar Rice Burroughs Tarzan books get progressively more bizarre as they go on (the number of "lost cities" one can find in Africa is truly staggering), there are still some pieces that could be used to build a sequel to this classic adventure story.
The Son of Tarzan
The Legend of Tarzan ends with the birth of a son for Tarzan and Jane. In the books, the pair also has a son, Jack Clayton, who, as a boy, finds himself lost in the same jungle that his father did. While the story was a bit of a retread in the original novels, the fact that Legend of Tarzan had no proper origin story means that they could use the young son to tell the story they skipped the first time around. Perhaps Legend of Tarzan starts so late in John Clayton's life because it's only meant to be a prequel for the story of the son. Either way, we'd fully expect the child of Tarzan and Jane to become a regular part of any sequel.
Opar, the fictional part of the Congo seen in Legends of Tarzan, is a place that was created for the novels. Edgar Rice Burroughs writes Opar as the remnants of the lost continent of Atlantis, which is probably a bit too wild for the "based in reality" story the movie tells. However, there are other characters that make appearances there who can be used. La is the priestess and ruler of the ancient tribe who lives in Opar, and one of the few other significant women who appears in the stories. La falls in love with Tarzan, but as Tarzan loves Jane, he does not return these feelings. As such, La can be either an ally or enemy depending on how they want to portray her. If they go here, they should probably not go the Burroughs route of making the tribe full of normal, human looking, women, and ape-like men. Just a thought.
If we're going to add anything more fantastical in the Legend of Tarzan films, it should be something more in line with the already fantastic main character. He has a couple of recurring animal sidekicks in the books, and while most are also primates, and thus already less interesting because the stories are full of that, one is a golden lion. Jad-bal-ja is a lion that Tarzan raises from a cub and who becomes devoted to the lord of the jungle. If a sequel to Tarzan can get their special effects budget under control in order to give a character like this a proper look, it could make for a fun new character.
One of the interesting things that Legend of Tarzan does is place the movie in the middle of real-life events, including the addition of Samuel L. Jackson's George Washington Williams, a real-life person who was truly responsible for helping to end atrocities in the Congo. If we're going to continue to place real life people in Tarzan movies, might we suggest Theodore Roosevelt? He's one of the most interesting characters in all of history, and, as luck would have it he, in fact, traveled to Africa himself. In 1909 and 1910, as part of a Smithsonian expedition, Roosevelt went on a Safari to the east and central Africa, including parts of the Belgian Congo. What will these two characters do together? We have no idea, but it would surely be fun to watch.
World War I
If we want to set the next movie a little bit later in time, or set a third film following a successful sequel, World War I is right around the historical corner. It also played a part in Edgar Rice Burroughs Tarzan the Untamed. In that book, German soldiers attack John Clayton's plantation and kill several of his friends including, apparently, Jane Porter. Tarzan then gets a little vengeful. While we would recommend not adapting the book exactly, the novel treats Germans about as well as earlier stories treated Africans, the time period is absolutely one of conflict, and some sort of story can certainly be told here.