According to prominent leaker Lance McDonald, Elden Ring was initially set to receive two major DLC expansions, but plans since changed and now we're just going to be getting Shadow of the Erdtree.
McDonald outlined on a Reddit post that he believed the first expansion was meant to be centred around the colosseums dotted around the game's map. These were given some focus in the major PvP update for Elden Ring last year, but we could have reportedly had more content.
At least we know that one major expansion is coming to Elden Ring, and hopefully with the colosseums content out of the way, we can get more polish put into the highly anticipated Shadow of the Erdtree expansion.