Well, Dark-San is contemplating Fiends. Even Swampert seems keen on the Fiends. Nace uses Fiends.

So here's my stab at a Fiend Deck.

Monsters (19)

Slate Warrior x 3
Archfiend Soldier x 3
Dark Necrofear x 2
Night Assailant x 2
Kuriboh x 2
Dark Ruler Ha Des
Jinzo
Sangan
Cyber Jar
Breaker the Magical Warrior
Tribe Infecting Virus
Sinister Serpent

Magic (13)

Dimension Fusion x 2
Book of Moon x 2
Pot of Greed
Graceful Charity
Card Destruction
Snatch Steal
Enemy Controller
Creature Swap
Lightning Vortex
Premature Burial
Swords of Revealing Light

Traps (8)

Compulsory Evacuation Device x 3
Mirror Force
Torrential Tribute
Ring of Destruction
Call of the Haunted
Sakuretsu Armor