I'm currently looking for the following suggestions:

• Arena Style (not sure if I want interactive objects on the arena or not). Something like jungle, desert, etc.
• New enemies, with unique abilities or behaviors.
• Interesting weapons. They can be normal everyday weapons or very cool ones with interesting functionality.

Suggestions

Arena Style

• Ancient Ruins - Desert
• Ancient Ruins - Jungle
• Ancient Ruins - Tropical
• Colosseum
• Battlefield - Grassy Plain
• Village
• Burnt Village
• Ship (Pirate or something, could be made to look pretty cool)

Enemy Types

• Archers - Ranged
• Crossbowman - Ranged
• Brutes -  Heavy hitter, but slow windup
• Knight - Medium Damage, Medium Speed
• Horseback Knight - Fast and Medium Damage, but vulnerable to thrown weapons and trap weapons (more on that later)
• Barbarian - High Damage, High Speed But Dumb, Uncoordinated AI
• Wild Beasts - Lions, etc
• Monsters - Ogres, Goblins, etc. (If planning to go a more fantasy route)

Weapon Types

• Javelin/Spear (Thrown weapons)
• Caltrops (Anti-Horse weapon)
• Katana (Quick, sweeping hits, maybe a rare item)
• Flail (Would be cool to implement with the physics)
• Rope Bow (Pull Knights off of horses, probably other uses)
• Tactical Nuke
• Great Sword
• Chakram
• Magic? (If you want to keep it in the realm of reality, maybe not the best choice)
• Staff
• Flintlock Pistol
• Greek Fire (Environmental Hazard)
• Bow (With the potential to shoot an arrow through fire, making a fire arrow.)
• Throwing Stars
• Sword + Shield

There ya go buddy. That's all I can think of at the moment.

wow. that is a great list of suggestions. I'll be adding a lot of them to the list!