It can be sreious or silly. Here is my whack at it.
The Infinity is flying away from Requiem BUT ALL OF A SUDDEN a whole armada of Rainbow Bunchies flys in and is about to attack the Infinity,then Master Chief says “I know now what I must do” after that he gives Cortana to Lasky or Del Rio and he gets in a lifeboat with a huge bomb and flys into the middle of the Rainbow Bunchie armada (The Dead Island Theme is palaying in the background), Halo 5 and 6 are about what happens durring his stay on Requiem leading up to the Rainbow Bunchie attack
TL;DR Rainbow Bunchies are the Main Enemy
Now its your turn 
(let your Halo creativity soar)
Noble team rises out of the fog.
Chief is running towards the Infinity to escape Requiem with it when all of a sudden… KAT FROM HALO REACH COMES IN A WARTHOG AND RUNS THE CHIEF OVER!!!
And then you see the Chief’s squished armor on the floor in front of a fireplace as a bear rug for someones library.
> Noble team rises out of the fog.
…and Kat pilots the Infinity into the sun.
343 takes a leaf from just about… EVERY MOVIE EVER MADE and Cortana and Chief kiss. Then they ride off into the sunset in a Warthog.
EDIT: -Yoink!-, someone beat me to a Kat joke. I’m losing my edge…
Cortana accidentally releases the didact and he uses the composer to become a AI monitor (like 343, because he is dying)Then leaves to start a new army while chief goes after him with the arbiter. Legendary ending the chief and arbiter are sipping drinks on a ship and arbiter spills his drink since he has no cheeks. The end. 
> > Noble team rises out of the fog.
>
> …and Kat pilots the Infinity into the sun.
>
> 343 takes a leaf from just about… EVERY MOVIE EVER MADE and Cortana and Chief kiss. Then they ride off into the sunset in a Warthog.
>
> EDIT: -Yoink!-, someone beat me to a Kat joke. I’m losing my edge…
Yay, I finally make a Kat joke before anybody else!
Turns out PVT Jenkins is behind everything, and is the leader of the Precusors. The newly fused 343 Guilty Spark/MC have to rally whats left of the Storm and Covenant to face the true threat.
An oversized Kool-Aid man falls from the sky “Oh NO!” Chief yells"Oh YEAH!" Kool Aid man yells and everybody dies the end
the chief goes into the center of requiem and see’s johnson
MC: wait! I thought you were dead
Johnsons: same here, same here
My ending is the legendary ending.
Oh…whoops…sorry…spoilers 
A grunt wakes up, and it was all a dream.
Master Chief’s clone survived and joined the enemy long before the Chief set foot on the first Halo. His evil clone shows his face at the end of Halo 4. He looks like Bill Gates.
its all like ckief pulls out cortanna’s chip out of his helmet and he tries to kiss cortanna but chief keeps on smashing cortanna’s chip into chief’s helmet were his mouth is until she is broken.
i meant to write smashing
343 hires the Bioware writing team and makes the ending so artistically unique, only they can comprehend it.
A multitude of epic events happen that seem to lead up to an epic final confrontation that will tie up many loose ends, satisfy millions of Halo fans, set up the story for Halo 5, and have Halo forums blowing up with OMG’s and HOLY CRAP’s… but instead the credits roll. XD
The infinity escapes Requiem, but it hits a glowing space rock!!??
No, they stumbled upon an ever armor locking George, and the Infinity gets EMP’d fo life.
You control a Halo IN GAMEPLAY as a “turret” to scorch the surface of Requiem.
(Got the idea from the leaked menu of Requiem with a Halo orbiting it.)
> A grunt wakes up, and it was all a dream.
lol