More card ideas from the darkdriver...but first, some other ideas.
How about basing some lands on the environment found in YUA? the five most common sites are: the Bokuto precinct itself (W); Natsumi & Miyuki's apartment building(U); the nearby school(G); construction sites(B); and stretches of highway(R).
Better yet...how about places in NeoTokyo from Akira?
The lands could have a triggered effect based on using the mana to cast creature spells (W grant's the creature cast a +1/+1 counter, R grants haste, B destroys a creature of the same color as the creature cast, and so on).
Anyway, onto the card ideas!
Urd's Game--instant
4UU
Starting with controller, all players roll a die. Then each player reveals that card from his/her deck. ("1" means the top card, "2" means 2nd from top, etc.) If the revealed card can be put into play, it comes into play; an "x" in the casting cost equals 0. If it can't be put into play, that player loses the game.
"Is it like Monopoly?" "More like Life..." --Urd and another player of the game.
(From Oh My Goddess!)
Dai-Guard--legendary artifact creature
10, 8/8
If Dai-Guard attacks or blocks, put a -1/-1 counter on him at end of combat unless you sacrifice a land.
During your upkeep, you may pay 10 to remove a -1/-1 counter from Dai-Guard.
"That pilot's going to bankrupt the company!"--the ceo of the company that owns Dai-Guard.
(From Dai-Guard)
Go Dynasty Relics--artifact
10
Tap: add x colorless mana to your mana pool. You choose x.
If you suffer any damage, instead prevent that damage and sacrifice Go Dynasty Relics instead.
"Not the relics! They're worth more than you are!"
(From Assemble Insert)
Natsumi's Brakes--instant
2W
Play only during combat. Tap target creature you control: untap target attacking creature you control. That creature neither deals nor receives combat damage.
"There go another pair of shoes!"
(From YUA)
Officer Yoriko--legendary human policewoman
1W, 1/1
Prevent all damage done to and done by Officer Yoriko. You choose the outcome of all coin flips and dice rolls. During your upkeep, all players reveal a card from their hands.
"You little gossip hound! How'd you graduate at the top of your class?" "Just lucky..."--Natsumi & Yoriko.
(From YUA)
Officer Chie--legendary human policewoman
3W, 2/2
When Officer Chie comes into play, choose target opponent. That opponent chooses targets for all instants, sorceries, and local enchantments you play while Officer Chie is in play.
Tap: add x mana of any one color to your mana pool. You choose x.
You have no maximum hand size.
"I'll show everyone that I deserved to graduate top of our class, Yoriko! I swear I will!"
(From YUA)
And of course...the Colorful entries...
Giant Ko-gal--giant schoolgirl
4U, 5/5
When Giant Ko-gal comes into play, tap all creature cards with pictures of men on them. They don't untap during their controllers' untap phases.
If Giant Ko-gal is equipped, sacrifice a land when Giant Ko-gal becomes tapped.
"Tell me you got that on tape!"--Anchorman to a cameraman.
Grand Prize--artifact creature
10, 5/5
Cumulative Upkeep: 1
Must pay 5 to attack with Grand Prize.
If controller's life is less than Grand Prize's power, remove Grand Prize from the game.
"Come on, get me those diamonds!" "I think this card hasn't been maxed out yet..."
Consolation Prize--artifact creature
10, 1/1
Indestructible.
If Consolation Prize goes to the graveyard, its controller loses life equal to its toughness.
"Look, I made you a sweater! Let's go out and show it off!"
And finally...Ah...regulation. So that means no ante effects, no card-flipping effects...and horsemanship is right out.
And here I am with 20+ decks that include Banned-6 cards.

[Banned-6 cards are cards exclusive to the 6 banned sets: Portal 1, 2, 3, Starter, Unglued, & Unhinged.]