Tommen can't do that as he is the King as well, which remains a Baratheon. If you read history of House Lannister, this won't be first time that House Lannister continued from female line. Male direct line of House Lannister died out long ago when King Gerold III Lannister died without a son, his grandson from his only daughter, Ser Joffrey
Margaery tells Tommen he should attend court and listen to petitioners, and the boy king starts to openly object to Cersei's hatred for Margaery. The young queen gains favor with the residents of King's Landing by visiting local markets to buy fresh fruits, bread, and fish, ordering dresses from local seamstresses, and making great public shows In the wake of the Sept of Baelor's destruction, Cersei's actions prompt Tommen to commit suicide by plummeting from the Red Keep. Cersei is visibly less shaken by Tommen's death than she is by Joffrey and Myrcella's. The queen calmly observes her son's body before immediately moving to seize the throne. Moments before the rape, king-to-be Tommen (Dean-Charles Chapman) got a history lesson from grandpa Tywin (Charles Dance) on the nature of being a good king.Tywin’s speech showed that he really Tommen under Cersei is bad. Tommen under Cersei leaves everything to her and is made a child again. Tommen has the potential to be a great king. Not a good king, not a better king than his father and brother, but a GREAT king that can potentially reform the Kingdoms into a better system.With little hesitation, Tommen subsequently removed his crown, climbed onto a window ledge, and leapt to his death, giving literal meaning to the name “King’s Landing.” Tommen’s suicideFjzP.