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.
looking at list of the cards to be expected so far already looks bad, Bowganian Nightmare Steelcage and Dark Room of Nightmare creates 2000 damage, plus Spy to summon Cannon holder and another gravekeeper in hand with Dark Room of Nightmare equals another 2000 damage, pretty much torture, obviously fitting Yami Marik's style if that is corrected.
So if they really add him in the game (and I'm pretty sure they will), then he's probably going to be a non-playable character only and will appear in an event just like Yugi Muto and Mokuba. Makes me kinda sad because he's my favourite character, so I really hope this assumption is wrong.