Kingdom Hearts III Theory about Eraqus's Keyblade...

Yes
I think a bigger question is
without a keyblade
what happens to Aqua

 

I wonder about this as well. Why would Aqua do that if she knows that she's helpless without Eraqus' Keyblade. Although she can use magic, she usually channels it with the Keyblade. So without it, she can't use any of her abilities.

 

dun dun dun
its a good possibility. Maybe Aqua lets herself get consumed by darkness in order to help out Sora in some way or another.

 

We have to remember, it's a concept trailer for a game that was just announced.

Either way, it probably ended up there with Aqua a bit aways. It's like 2 sides of the same coin, same reason why Kairi's letter ended up in the dark meridian.

 

I'm not too worried about that. As far as we've seen, the beach in Realm of Darkness is clear of Heartless, and last time we saw Aqua (BbS Secret Ending) she was safe there. My theory is that if Sora and Riku got out with a door to the light, Aqua can do the same, along with Eraqus' Keyblade.

 

It's probably just a simbolic way to show Sora's gonna save Aqua, or it's simbolic on a Meta level and it simbolizes the fact Square is picking up where they left off.
Either way, it's a concept vid just so people get hyped, it'll probably not even happen in-game.

 

I don't think it's anywhere near that symbolic
We pretty much KNOW Eraqus' keyblade will be important because it's he only keyblade that can solve Castle Oblivion

 

...no.
Master's Keeper got nothing to do with the Castle Oblivion puzzle. It can turn LoD into it sure, but only Aqua's memories can get you to the right room.

 

I agree with Red. I think it's suppose to symbolize maybe Sora becoming a Master or that there will be some kind of connection between Sora and Master Eraqus.

 

... and by extension is can turn Castle Oblivion back, or SOMETHING
Obviously the keyblade itself is important, or they would have just used Aqua's keyblade

 

They don't need to turn Castle Oblivion back though.
You probably need to find the Chamber of Awekening to turn it back, since the lock was on the chair Ven's sitting. So if Sora got the Keyblade and somehow knew how to use it to turn it back to LoD to skip the requirement of having Aqua's memories, he wouldn't be able to anyways. So, Master's Keeper isn't important.

 

If the Master Keeper isn't important, why did they bother to make a 3D model of it this early?

 

Because it represents Square itself?
I mean, Square handle, E(nix) shaped teeth, original owner's the guy whose name is a anagram of Square and has the same face as FF series's creator, etc.

 

... seems really unlikely