Mare of Easttown was always intended to be a limited series (and it will compete in that category at the Emmys). However, it wouldn't be the first limited series on HBO to get another season, since Big Little Lies already paved that path.
And the show is called Mare of Easttown, after all; the title isn't tied to the first season's central case (not even "Murdur Durdur"). Creator Brad Ingelsby is open to exploring a second season.
"It was written as a limited, and it ends — there’s no more mystery to be solved. Kate and I, if we could crack a story that we were really proud of and felt like it was a deserving second chapter in Mare’s journey, then maybe," he told The Hollywood Reporter.