Marik Ishtar is a character that's suspected to be coming soon. There are several files found in the game showing a character selection icon, and a player profile icon. Similar to Yugi Muto, he has an alter ego called Yami Marik. His Yami version is more likely to be an unlockable character to avoid having 2 of the same character. Marik Ishtar might make an appearance through a mini event just like the Yugi Muto event.
Based on the anime, Marik Ishtar takes control of other people and use their decks to duel and does not duel himself, unless he is in his Yami Marik form. If he were to appear, he could be bringing other gravekeeper monsters like Gravekeeper's Descendant, similar to Ishizu and Odion's duel rewards.
I'm doubtful over Gil Garth's introduction since that provides Yami Yugi with a 2K normal beater while he already has Neo and Diskblade Rider. I just want some better normal dragons for my man Seto. I don't need Luster Dragon #1, just give us something to make Mai not outright superior unless we have triple Stamping ;_;