The format for the 2016 Video Game Championship season was announced on the official Pokémon website on December 9th, 2015. As of January 1, 2016, players competing in Play! Pokémon video game competitions will need to follow these new rules and regulations when building their teams and during battle. This will be the official format for the 2016 Pokémon World Championships in San Francisco, CA.

Basic 2016 Video Game Championship Format Details:

• All battles will be Double Battles played in Pokémon Omega Ruby and Pokémon Alpha Sapphire.
• Time limits for battles will vary by event. At major events (Regional Championships and above), battles will have a 15-minute time limit, 45-second move timer, and a 90-second Team Preview.
• Pokémon brought in via Poké Transporter will not be permitted (the blue pentagon is required).
• All discovered Pokémon will be allowed except the following Mythical Pokémon: Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, and Hoopa.
• Teams may have no more than two of the following Pokémon: Mewtwo, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal, and Zygarde.
• All items will be allowed except Soul Dew. Each Pokémon must hold a different item.

Source: http://www.pokemon.com/us/pokemon-news/the-2016-video-game-championship-format-has-been-announced/