I'm, of course, talking about the giant green prism looking thing in the corner. I'm trying to figure out what it could possibly be, and my only thought is something similar to the Cornerstone of Light. 

 

 

I have no idea, though, so I'd love to hear all your thoughts.

It's not the Cornerstone of Light, we know for a fact it's still where Disney Castle is being built.

 

My idea is it could be used as power source or a means of communication similar to what Master Eraqus uses to speak to Master Yen Sid in Birth by Sleep.

Yeah, sorry if that was misleading, but I don't think it is THE Cornerstone of Light, but maybe something like it, something of equal or greater power.

It's the Sims! Kidding, all honesty I believe it has something to do with the world,perhaps the heart of the world, or a portal in which you enter the room the two are in (want to say that's definitely riku). I think it's purposely shown in the video however, so I do think it's an indication that's it's not simply just decor.

It's the Sims! Kidding, all honesty I believe it has something to do with the world,perhaps the heart of the world, or a portal in which you enter the room the two are in (want to say that's definitely riku). I think it's purposely shown in the video however, so I do think it's an indication that's it's not simply just decor.

I'm assuming you haven't played every game in the series, but I can assure you that is not Riku

It's the Sims! Kidding, all honesty I believe it has something to do with the world,perhaps the heart of the world, or a portal in which you enter the room the two are in (want to say that's definitely riku). I think it's purposely shown in the video however, so I do think it's an indication that's it's not simply just decor.

The guy you seem to think looks like Riku (even though the only resemblance is hair colour) is actually Young Xehanort who is training to become a Keyblade Master and acquire one of the most ancient keyblade of his time.