Stephen King is the modern master of horror, but his range covers everything from psychological terror to small-town drama. If you have read through his catalog, these authors write with the same unsettling eye for ordinary life gone wrong.
Both write accessible horror rooted in small-town American life. Koontz tends toward more heroic protagonists and less nihilism, which makes him a good next step if King's darkness sometimes goes too far.
Both write horror that takes character seriously. Straub is slower and more atmospheric - he co-wrote The Talisman with King, and their styles complement each other well.
The apple did not fall far. Hill has King's gift for character and pacing but writes tighter, more focused stories. If King sometimes sprawls, Hill stays lean.
Both write horror grounded in family dynamics. Tremblay leaves more to interpretation than King does - his scares come from what might have happened rather than what definitely did.
King has cited Jackson as a major influence. Her horror is quieter and more domestic than his, built on social anxiety and creeping dread. If you want to understand where King came from, start here.