Ever since the end of the first season, "One Punch Man" fans have since concluded that the second season will give Saitama the challenge that he desires. Manga and Web Comic readers have concluded ever since that who would be the perfect enemy except Garou.
Also, other rumors claimed that Lord Boros would come back from the dead to take revenge from Caped Baldy. But who would win if Garou would fight Boros?
According to One Punch Man Wiki, Web Comic creator "ONE" stated that Lord Boros VS. "Perfect Garou" would be "one hell of a battle". If based on "One Punch Man" manga created by Yusuke Murata, Lord Boros would surely be able to defeat Garou.
But when "Perfect Garou" will fight Lord Boros, that is another story. And since the real core ability of Garou is his insightful view of his enemies, he could stand a chance against Lord Boros.
If Lord Boros will come back from the dead on "One Punch Man" season 2, and should he meet Garou, it is a great possibility that Garou will fight him. And since Garou will always get strong in every enemy that he faces, and this can happen even while on a match, Garou is theoretically the winner. The guy just keeps improving and improving.
And since Garou, like Saitama, broke his "limiter", he too can have a limitless power. It is not because Garou has greater power than Saitama, but his ability to exceed Saitama's power when he's been cornered. This makes Garou beating Saitama possible on "One Punch Man" season 2.
And as per Saitama, it is possible that he can also have a romantic interest in "One Punch Man" season 2 according to Droid Report. But it seems like it would be a one-sided interest. Tatsumaki is the only notable female character from the first season, but it is expected that her sister Fubuki would make an appearance on the next season. And for a little spoiler, Fubuki is obsessed with Saitama. Wanna know why? The answer would be revealed on "One Punch Man" season 2.