Adidas AG’s next chief executive officer is set to inherit an even tougher job. The company warned Thursday that unsold goods are piling up as consumer demand weakens across China and western markets. That sent the German sport apparel maker’s stock slumping as much as 9.1%. Problems are accumulating under outgoing Chief Executive Officer Kasper Rorsted, including a mounting publicity crisis over an alliance with rapper and designer Kanye West. Whoever replaces Rorsted as CEO next year will need to come up with buzzy products as the future of the bestselling Yeezy line is in doubt. Adidas shares have now...