Quite simple here is mine:
The ‘Zealot’ Skull
All enemies are now Zealots with 2x shielding as well as increased rate of fire, movement, and grenade/weapon accuracy
How about you?
The Campfire skull
’I like the smell of roasted Grunts’
When meleeing an enemy, he will be lit on fire having a slow and painfull dead…
The Haunted skull
’I see dead people’
A mist clouds the battlefield, all enemies have poor Camo and there is no music during the campaign, only the wind and the cries of the lost and -Yoink!-…
The Stinky skull
’What? Do I smell bad or something?'
No enemies will come any closer to you then 10 meter + pestilence AE is on…
Blue Ball skull
The only weapon available for use is the plasma grenade (the same goes for enemies) and you have an unlimited supply. Enemy rate of tossing increases by 3 fold
Tick Tock
"Is this a race?"
Activating this skull puts a thirty minute time limit on each level.
The Friend or Foe skull
finding this skull will turn all your enemies against each other
super pistol - BOOOM!
magnum shots blow up on unshielded targets
> The Campfire skull
> 'I like the smell of roasted Grunts’
> When meleeing an enemy, he will be lit on fire having a slow and painfull dead…
>
> The Haunted skull
> 'I see dead people’
> A mist clouds the battlefield, all enemies have poor Camo and there is no music during the campaign, only the wind and the cries of the lost and -Yoink!-…
>
> The Stinky skull
> 'What? Do I smell bad or something?'
> No enemies will come any closer to you then 10 meter + pestilence AE is on…
I love how you made a little quote for each one
The Treason Skull
All friendly AI turn on you if you damage any of them, or fail to protect them adequately.
Heretics
All enemies wield human based weaponry and drive human based vehicles
TWEET TWEET
dude(what,) I cant feel my legs
every foe turns into a engineer and fly`s into a the sky, just to transform back to there original state.
(There for, they fall to the ground and die)
Easy legendary 
This one came to me in my sleep, of all places.
Population Overload Skull: Double or Triple the enemy numbers on any Campaign mission.
Easy - 4x pop.
Normal - 3x pop.
Heroic - 2x pop.
Legendary - 2x pop.
> TWEET TWEET
> dude(what,) I cant feel my legs
>
> every foe turns into a engineer and fly`s into a the sky, just to transform back to there original state.
(There for, they fall to the ground and die)
Easy legendary
Haha, i like this one very much ! But you would need to do something out of this world to get it 
Also maybe the quote could be
TWEET TWEET
Dude, Where’s My Legs? (reference to “Dude, Where’s My Car?”
Every foe turns into a engineer and fly`s into the sky, just to transform back to there original state
Halo CEA skull:
Fortune Teller Skull
’We’re back from the future’
Enemies and weaponry from later Halo games also appear in CEA’s Campaign
Slow Skull
Wait, where’d you go?
Increase enemies speed by 2 and decrease player’s by 0.5
EMP Skull
[LOST CONNECTION…]
When hit by a charged plasma pistol shot, HUD disappears for 10 secounds and can’t move at all for 5 seconds.
Anti-Animal Skull
All I was doing was destorying your habitat!
All wild creature turn on you and attack you.
Zookeeper Skull
Sick 'em!
All wild creature aid you in your assalt on the enemy.
Just some ideas…
Nobody Likes a B*tch
"Don’t mess with me…"
PROS:
Bottomless Clip
Infinite Jet Pack
Super Speed 300%
Super Gravity 50%
Full-Auto Target Locator
You can Forge in Campaign
CONS:
You have to use the Xbox 360 Kinect to control your player.
"Sucks to be you…"
The Bobble Skull (Secondary)
Swell headed, indeed
(all A.I.s get 4x bigger heads)
The Inconceivable Skull (Primary)
This skull makes headshots instakill and cuts your health down 25%
The Easter Skull (Secondary)
More Easter eggs are unlocked, so while the skull is active, you can find tons more easter eggs!
The Brutal Skull (Primary)
All Brutes go 2x fast, have 2x health, 2x power, and there are 50% more brutes
(That means take half the brutes in the game, double it, then add half of it(I think I over explained that T.T))
Viewpoint
Allows you to play the game in 3rd person.
Strongarm
Amount of gernades you can hold becomes 4 for each type, and the distance you can throw them is doubled. However, enemies get the same privelage
Energize
All enemies have a 2x overshield
Fireworks
When you kill an enemy that is using a jetpack, there pack malfunctions, they fly up in the air, and explode in a bright colorful firework like explosion.
Triggers Down!
Hot barrels, indeed!
Allies and enemies fire as rapidly as possible, when engaging in combat. Their weapons don’t overhead or need to be reloaded.
The Greenskull
Your only weapon is a golf club. B melees, right trigger fires a giant golf ball.
Haha, that’s a good one! xD