BELGRADE, Serbia—Novak Djokovic returned home Monday after being thwarted from defending his Australian Open title only to face a new predicament: He could be barred from the French Open this year, too, if he’s still not vaccinated against COVID-19. A plane carrying the No. 1-ranked player touched down in his native Serbia, closing at least the first chapter in a dizzying drama that has resonance in the world of elite sports, Australia’s pandemic politics, and the polarized debate over the coronavirus shots. A handful of fans waving the Serbian flag greeted him at Belgrade’s airport. Djokovic has an almost iconic status in Serbia, and many there felt he was poorly treated by Australia. But his troubles may not be over yet: He could be barred from the French Open this year, under a new law intended to exclude the unvaccinated from stadiums and other public places. Much could change between …