Caesar had no legitimate heir upon his death to inherit his estate or considerable influence through his name. His daughter Julia had died about ten years earlier and Caesarion was not recognized as his heir under Roman Law. I should also note that there were questions about Caesarion's true parentage. Despite Cleopatra's claims and the fact that both were guests of Julius in Rome until his death in 44 B.C. he never officially acknowledged Caesarion as his son. Not only that but Caesarion was young in at the time of Caesar's death, barely an infant. He had no idea who or what this child would grow up to be and that meant passing on all of his influence to Cleopatra. Giving that influence to the queen of a foreign client kingdom was almost certainly something that wouldn't remain unchallenged by Caesar's political allies let alone his political enemies. I can only speculate as to why he chose Octavius as his adoptive son, but I'll point too two crucial aspects. First Caesar knew him quite well as Octavius was raised for several years by his grandmother who was Caesar's sister. Octavius also served briefly with Caesar in Hispania where he managed to leave quite the impression upon him. It was after Caesar returned from his Hispania campaign that he deposited his new will which named Octavius as his primary beneficiary. Secondly, Octavius was someone who would be accepted however begrudgingly as his heir. He was Roman, distantly related (grandnephew), and he was 19 which meant he could directly inherit everything Caesar left to him. While Antony and his supporters certainly questioned and attempted to belittle and dismiss Octavius claims to Caesar's fortune and political capital the anti-Anthony camp quickly seized hold of the young heir as a way to rid themselves of Antony. I can't say whether this was all part of Caesar's plan and using someone young enough to be seen as non-threatening, but old enough to inherit his name was a brilliant masterstroke. I find it more likely Julius Caesar simply saw Octavius as the best possible choice and least controversial.