SoftBank Group Corp. founder Masayoshi Son aims to stay at the helm of his company longer than originally planned to focus on the burgeoning business field of artificial intelligence, but is now under increased scrutiny over his longer-term strategy after the surprise departure of his successor.

Son faces a number of challenges, including turning around U.S. mobile network provider Sprint Corp., a SoftBank group company. And most recently, the abrupt resignation of SoftBank President Nikesh Arora, the heir apparent credited with making bold overseas investment decisions, has raised uncertainty about the conglomerate's investment business, analysts say.

Arora's departure, coming less than two years since he joined SoftBank from Google Inc., caught many off guard, putting the company's succession planning back to square one.