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.
Marik's skill: Sexiness: Once during a duel after your lifepoints are 1000 or lower, your opponent will be frozen by Marik's good looks and unable to attack, summon, or activate spells/traps. Mokuba's skill: Big Bro to the rescue!: Once during a duel when your lifepoints are 100 or less, Mokuba can call Seto to bring his Blue-Eyes to the field. (Yeah, I can dream. XD)
Unless the "mysterious tower" they referred to in the upcoming updates news is referring to Kaiba's Alcatraz tower where the Battle City finals took place, I don't think it would happen in September. Maybe in December? That way, we could all have a very Marik Christmas if he is unlockable in the event. XD