I loved this! And it definitely feels like kingdom hearts with a few new touches here and there. I would like to see your concept for the world map!
I agree with Beløw, because this is the end of the heartless/xehanort series, so I wouldn't really want anyone new to pop up unless it adds to the storyline. So, thinking about the main characters we already have, I think it would be nice if Rinoa would pop up for Leon...just for his sake. Or maybe they can do that in the end credits or something. But I am excited for what they might do with Lightning.
Definitely agree with Plums. 13 is a big number, but it's more prominent in KH2, while the 3 is the main, reoccurring number that shows balance between the characters and the whole concept of light, twilight/dawn, and darkness in all of the games.
I think I would definitely like to see Olympus Coliseum again just because having tournaments is just a great side quest thingy. I would like to see more of Disney Castle and the area around it, kinda like Epic Mickey where we can see the towns and such. I think there is a lot of potential for Port Royal because they can bring in Davy Jones and the Kraken. A lot of the main worlds, like Destiny Islands, Traverse Town, Twilight Town, Radient Gardent...and maybe a deserted World That Never Was...which could be a place that could give clues about the still lingering existence of Xemnas....ooh ooh, and the Keyblade Graveyard!
So excited about Kingdom Hearts 3 and Final Fantasy XV!