Well, here goes:
Title: Ultimate Kingdom Hearts
Platform: PS3(don't know why...)
Plot: A retelling of the whole kh story, with added battles, characters, and worlds. Begins in BBS, telling of that story that story that is yet unknown. Fades to kh1, with Sora playing through his part. Then goes through CoM, 358/2, and kh2. Finally, it goes through Coded and a new, extra plot piece.
One big difference includes the ability to play as Sra, Riku, Roxas, Aqua, Terra, Ven, Data Sora, and Mickey.
Gameplay: basically, the gameplay is the same as every other kh game, with it being slightly unique to each part.
Worlds:
BBS:
World of nothingness
The place where you battle terra in kh2fm
Sleeping beauty
Olympus coliseum
Pride lands
Destiny Islands
Hollow Bastion
Neverland
Alice in Wonderland (inevitable)
Halloween Town
100 acre wood
KH1: (all fm extras are here, too)
Destiny Islands (able to visit main island now)
Disney Castle
Traverse Town (New 4th, 5th, and 6th District)
Wonderland (able to meet Mad Hatter)
Olympus Coliseum (New tournaments)
Deep Jungle (More Tarzan back story, more tree sliding)
Agrabah (MUCH bigger town)
Halloween Town (More Halloween...)
Neverland(Ship, London, and Island)
Monstro (more battles)
Atlantica (few new areas)
Toy Story (yeah, it had to be.)
Hollow Bastion (More boss battles, including a version of Pete.)
End of the World (More ansem battles)
100 acre Wood
CoM: (with RE:CoM extras and 3d)
Path to castle oblivion, with fight hooded man... plays like kh1
Castle Oblivion (Olympus coliseum now has tournaments, and sephiroth battle. Deep jungle and toy story are available on floors 4 1/2 and 9 1/2.)
Not much more to say..
358/2:
Castle Oblivion
The World that Never Was
Wonderland
Twilight Town
Hercules Coliseum
Deep Jungle
Toy story
Pride lands
Hollow Bastion
Disney castle
Kh2: (including fm features.)
Traverse town( more Roxas days, bigger town)
Disney Castle( Way more castle)
Hollow Bastion(More castle... More city)
Timeless River
pride Lands
Mulan
Olympus Coliseum
Beast's castle
Toy story
Space paranoids
Halloween Town
Agrabah
Atlantica(Actual fighting...)
100 acre wood
The world that never was
castle Oblivion
the hunchback of Notre dame
Betwixt and Between(Whole world)
Data Twilight Town
KH Coded:
Whatever squenix puts there
New Adventure:
Destiny Islands
...
wow, long post.