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I have built a Berserker out to lvl 10 for an upcoming Dark Sun game. The concept is an Arena Fighter akin to the Dwarven Slayers from Warhammer (2 handed weapon, no armor, eat's face in battle). We're starting at lvl 1, but I built him out to lvl 10 for sanity sake.

Garoth the Bloodstained, Berserker lvl 10

Str 18
Con 14
Dex 16
Int    8
Wis 10
Cha 12

Feats:
1 Dwarven Weapon Training
2 Unarmored Agility
4 World Serpent's Grasp
6 Headsman's Chop
8 Spear Expertise
10 Improved Defenses

Powers:
A: Stalk and Strike
A: Run Down
1E: Batter Down
1D: Thunder Hooves Rage
2U: Shrug it off
3E: Brutal Slam
5D: Feral Rampage OR Silver Phoenix Rage
6U: Run Rampant OR Cull Weakness
7E: Curtain of Steel OR Deny Escape
9D: Eye of the Maelstrom OR Oak Hammer Rage
10U: Howl of the Alpha Wolf

Weapons:
Gouge

I'm leaving myself some room in 5-10 to fill either Striker or Defender roles as they come up. Also, a lot of the powers seem situational, so I'm waiting to see how the game progresses before making the final decision. Any suggestions for optimizing the build further?

DWT does give you Gouge, although it probably shouldn't from a balance perspective. 

One strong consideration for gouge barbarians should be fighter-multiclass and novice power to get Rain of Blows.  Three attacks on the same target is a huge benefit.  You have the dexterity for it.  If you're going to multi-class into fighter, the best option is Battle Awareness, but you do not meet the wisdom prerequisite.  You could get there now by dropping constitution and charisma, but that is a pretty hefty cost. Instead, I'd change you charisma bonus to wisdom, grab a different multi-class now, and then swap the multi-class feat once you get to paragon levels and you et the auto +1 to wisdom to meet the prerequisite.

There are also a number of great fighter class spear feats that work will with a barbarian that will do some charging, etc... 

If I were going to roughly follow your plan, my feats would be:

1.) DWT
2.) Spear Expertise (SE)
4.) Retrain SE -> Fighter Multiclass, Novice Power: Rain of Blows
6.) SE
8.) Unarmored
10.) Improved Defenses 
 

1) Rain of Blows is way better damage than Batter down.  Getting your modifiers three times makes a huge difference, and it's some striking you can do without dropping your defender aura.  Proning is good, but...

2) Standing does NOT provoke an OA, it just wastes the enemy's move action to stand up, so given that being prone makes them grant CA, pretty much every enemy will stand up when it can (unless you're doing KAM cheese or something else that triggers off knocking them down again).  Given that,

3) I think WSG is good if you're taking Run Down, because once you slow that enemy, whether it moves or shifts to get away from you, you can knock it over.  That's pretty solid at will proning (it takes two attacks, but since it will always interrupt their attempt to move, that's not a big problem).  Given that, you might not need an encounter power for proning.  If you want one, I like the level 3 primal one from PrP that prones, pushes, and does 1d8+mods to an enemy adjacent after the push.  Of course, being level 3, that precludes Rain of Blows...

4) I do recommend finding space for Berserk Vitality.  In encounters that you plan to spend mostly in striker mode, it's a solid HP buffer.  In encounters where you get beat down while defending, it can keep you alive for awhile when you flip the switch.