This is, of course, a rumor, but it should perhaps be taken a tad less seriously than other recent rumors. The Daily Star doesn’t list a concrete source, but apparently someone close to the show said that Pizzolatto is a huge fan of McGregor. Also:
“Nic will meet personally with Ewan and we hope to make an official season two cast announcement very soon.”
Bold words. McGregor would make a very interesting and worthy choice for season 2, so we wouldn’t be upset in the slightest if this turned out to be true.