Team Ico/WMG

Both the boy and the griffin will die at the end of The Last Guardian.
Because only one of them dying would be too predictable.
 * And not sad enough.

Both the boy and the griffin will live at the end of The Last Guardian.
Worked for Ico and Yorda!

The game maybe a prequel to Shadow Of The Colossus, so...
The griffin will end up becoming Dormin. I know it's far fetched, but the griffin does have horns...

Dormin was the boy and the griffin from The Last Guardian, combined.
Like the above WMG, but this troper predicted that in a climax of the game, the boy will do anything to save his friend (probably mortally wounded), so he decided to fuse himself with the griffin, suddenly becoming the local god in SOTC.

The boy in Last Guardian was intended to be a snack for Trico.
Would fit the other two games' fascination with Human Sacrifice.

Trico and the boy will survive
But become seperate somehow, possibly even believing the other to be dead. It would still fit the somewhat bitter end of the previous games, but not be as tearjerking as many have assumed thus far.

Trico will be killed by Wander
When the boy and Trico finally get to the top / exit of the ruins, they will arrive just in time to see Wander killing a bunch of people with the magic sword and hoisting Mono onto Argo. Then Wander will assume that Trico must be with the others and kill it. Whether or not Wander Would Hurt a Child and kill the boy too is unknown.