What do I do after the expert cup?

You run around until you find Ishizu (3 stars) and beat her afew times, if you did that you then battle Rishid. Afterwards you awaken Marik (that dude that said you were too strong named Namu? That's Malik, his 'yami' side is Marik) and have to beat him with the God card, but he is DIFFICULT so it'll take awhile.

This section of the game involves the Rare Hunters from the Battle City part
Of the Yu-Gi-Oh! Anime, and the story is similar. Once this part begins, you
Will begin to be challenged by Rare Hunters at night.

Event N: Find Ishizu Ishtar again, this time in the first city(she should have
about 3 stars). She will ask you if you prefer Unity or Power.
Regardless, you must face Seto Kaiba for an Egyptian God Card. If you
win, you receive a god depending on your choice. Unity gets you
Slifer the Sky Dragon, and Power gets you Obelisk the Tormentor. Yami
Yugi gets the one you don't choose. The Winged Dragon of Ra was
stolen from Ishizu, and she asks you to retrieve it for her.

Event O: After receiving a god card, you will start to be challenged to Shadow
Games at night more often by the Rare Hunters.

Event P: After a few duels with the Rare Hunters, look for a duelist icon with
no stars in the second city to find Namu. He will compliment you but
refuse to duel such a powerful opponent.

Event Q: Duel at night until you are stopped by Rare Hunter. He uses an Exodia
deck. Beat him, and his mind is controlled by Marik. Marik taunts
you, and Yami Yugi arrives soon after to warn you about other Rare
Hunters.

Event R: Look for Joey Wheeler (~4 stars) during the day in the second city.
He is under Marik's control. Win to learn more about Marik's plans.

Event S: Find Yami Yugi (~5 stars) during the day. Tea will appear, but she
is also under Marik's control. She will then leave. Look for her
(~1 star) and win to free her. Yami Yugi arrives to tell everyone
to look for strange people.

Event T: Find Odion (~3 stars) in the second city. He is pretending to be
Marik. He is one of the few Rare Hunters who does not stop you when
you are moving. He uses a lot of trap cards, so Jinzo and/or Royal
Decree are useful here. When you beat him, Namu reveals that he is
Marik, but Yami Marik (his dark side) takes control. Ishizu explains
how he became evil.

Event you: Find Joey Wheeler again in the second city (~4 stars). Marik explains
that he has a stronger hold on Joey's mind and that winning might
harm him.

Event V: Find Yami Yugi (~5 stars) in the second city. You inform him of
Marik's message, and he realizes that a tie would free Joey's mind.
He gives you two cards to help you force a draw: Ring of Destruction
and Self-Destruct button.

Event W: Find Joey once again in the second city. This time he agrees to duel
you, but reminds you that a win would harm Joey's mind/body. Make
sure that you drop BOTH player's life points to 0 at the same time to
force a draw. Winning or losing results in a Game Over. Yami Marik
appears afterwards to laugh at you, Joey, and Yami Yugi.

Event X: This event is available once W is completed. You have as much time as
you need to get more points, cards, and do whatever you want. Once
you feel that you are ready, look for a 5-star duelist at night in
the second city. This is always Yami Marik. Duel him in a Shadow
Game. You must reduce his life points to 0 using your god card to
complete this event. Winning normally gives you experience and
points, but you can't proceed until you strike the final blow to his
life points using your chosen god card. If you win without your god
card, Yami Yugi arrives and explains that you need to defeat Yami
Marik using your god card. After the credits roll, be sure to save
your game when asked if you don't want to lose your progress.

----------------------------PART VII - THE REMAINDER--------------------------
This part of the game is about collecting the rest of the cards.

Event Y: After the credits, you'll notice that Noah's base reappeared in the
water outside of the second city. You won't be able to enter it,
however, until you reach level 20.

Event Z: Once you get to level 20, the Duel Base reappears. However, this
time it is run by KaibaCorp. In it are three courses to try. Each
pits you against a few duelists in a row. You can enter the Duel
Base any time you want to try one or more of the courses.

Course 1 pits you against the Kaiba family. You must face all four
of them in a row: Mokuba, Noah, Seto Kaiba, and Gozaburo Kaiba.
Completing the course earns you 400 KCP and the Course 1 Clear
recipe.

Course 2 has five opponents that you must face: the members of the
Big Five. They use the same decks as before, so be ready to face
all five of them in a row. Completing the course earns you 500 KCP
and the Course 2 Clear recipe.

Course 3 includes most of the Rare Hunters. They are: Rare Hunter,
Arkana, Lumis, Umbra, Odion, and Yami Marik. Completing this course
earns you 600 KCP and the Course 3 Clear recipe.

Also, once you get to level 23, you receive two emails that tell you how to
Unlock the sound test and watch the credits. The sound test password is
57300000. The credits password is 00000375. You must pay 1000 KCP to use the
Passwords, but you will immediately get a full refund. Both only work once
You defeat Yami Marik.