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 am hoping that Marik will get his own original battle music. I can't see him getting the first two pairs you mentioned. (Wouldn't mind if he did, though. Cool tunes!) But I wouldn't be surprised if he got Ishizu's and Odion's theme. Hope that he doesn't though, because that BGM is sort of droney and sleep-inducing after a while.
Marik will be a Christmas event. His main card will be the Winged Dragon of Ra. That is also when the Egyptian God Cards will come out. When you summon them to the field the music will switch to the Egyptian God theme. Some of Marik's level up rewards are Juragedo, Newdoria, Makura the Destructor, and Gravekeeper's Visionary. Marik's event will be similar to the Bakura event. Collect dice and win. At 200,000 points, you will unlock Marik. Marik has specific dialogue with Yugi, Joey, Bakura, Kaiba, Odion, and Ishizu. There will be a bonus summoning animation for Slifer the Sky Dragon.