The prophecy states that one who survives Ra's blade shall be the new Demon's Head. If we fallow the literal translation there is only one who that fits the description. But a prophecy always have a double meaning, and should not be always taken so literally. From what I can tell there are three candidates
The Arrow is the most obvious choice with him being the demon's heir (at the moment). He fits in because he literally survived Ra's blade albeit with some help, and we all now what doesn't kill you just makes us stronger. So far this is Ra's choice to be his successor evan favoring him over his own daughter. However knowing what the league is at the core Oliver is resented to becoming apart of them until he is forced into it.
Maseo Yamashiro joined the league of assassins after the death of his son among all the league members he still seems to have connections to past life clinging to who he was far after his cleansing. Five years prior to the season he was Oliver friend and is forever in his debt for saving his family's life multiple times. In the past episode Maseo helps team arrow trying to sneak Oliver out of the catacombs. When Oliver returns he presents his sword to Ra's so he could kill him , seeing as sign of loyalty he spared Maseo's life. He survived the wrath of the demons head thus surviving his sword. Could this be a sign of his potential statue of being the next Ra's? If he were to become Ra's it would be a much more lighter and more honorable organization
The man personally responsible for Oliver's queens being stranded on the "Island". After Killing who he thought murdered his wife he left his son and his fortune behind and joined the League. When the time seemed right he left the league with Ra's al Ghul's permission (the first one in the league history) and under the undertaking killing 503 people. For several months everyone believed him to be dead, until Moria Queen informed Ra's of his living status and the league hunted him down until he forgiven by Ra's to gain Queen's favor of joining. Now this is my personal candidate to become the next Ra's, also has more fore him. Do you see it. On paper the League of Assassins is a body with Ra's Al Ghul being the head and the rest is the body (remember what queen said about the body being a weapon) As Malcolm being the weapon ( perhaps a sword) he left the order and pursued his own agenda, Another reason is that after the undertaking league went after him, he was pursued and caught, then tortured by Ra's and he Lived and is freed from the league. Could this decision come back to haunt Ra's, will his greatest mistake become his successor