Reasoning:
Naminé's special to him. He can't afford to lose her.
If Monty Oum's gonna add Sora, he should add Roxas too.
It's the Key of Destiny...his speed and aggresiveness, added to his wielding two keyblades whenever he wants
Reasoning
She's officially the main hero of Birth By Sleep
She created Castle Oblivion!
She whooped Xehanort in full power
She's speed, cartwheel, and magic oriented! How would that not be helpful?
She obviously would be an epic team member for Kairi and Namine to have, let alone anyone?!
Reasoning:
He can use both light and darkness like Riku
He can ride his Keyblade as a friggin' motorcycle! Take that, Cloud!
Look at what he did to Braig!
Reasoning:
Kairi can't be the only Keyblade wielder kicking butt, can she?
He is the Keyblade's Chosen one & he has a promise to Kairi that he will protect her.
He defeated Xehanort's Heartless, Half the Organization, Xemnas, and Sephiroth...twice! Who wouldn't put him in?
Reasoning
Kairi is already in there. There planing to put Sora. So Why not Riku too. To complete the Destiny Island Trio
Riku defeated half the Organization and defeated Xehanort's Heartless, and kept his heart when Xehanort's Heartless stole his body
He wields the power of darkness
Reasoning:
One of the main villains of Birth by Sleep
The Dark part of Ventus' Heart
Has a Really Cool Keyblade Called X Blade
Reasoning:
In a way, Ventus is the reason Sora is the hero he is today.
If Terra and Aqua appear, Ventus would complete the trio?
If Vanitas appears, then so should Ventus, as they are two halves of the same heart.
If Roxas appears, it would make a strange yet interesting turn of events, as the look exactly alike.