Kingdom Hearts 2.8... Wtf?

[Dr.Henry_Killinger]

Hiku said:

I wouldn't call DDD a spinoff either.
A spinoff game is fundamentally different in some way, mainly in gameplay and/or story.
The gameplay of DDD is very recognizable. Combat system is familiar.

spinoff is a crappy term, these are side stories aka non-numbered series. Familiarity is irrelevant, the core system - the command system - is that of BBS - and has never been in a Numbered series, thus it is experimental, Toriyama is trying out new things and perfecting the command system. Story wise it makes sense to, Aqua, Ventus, and Terra where Master Level, so the concept of an MP bar didn't make sense for them. 

* taking the mark of mastery, Sora and Riku now use commands cause they are that skilled now, essentially they have infinite MP.

* The only thing Riku and Sora can't do yet is transform their keyblades...

There's traveling between different worlds. And the main plot centers around Xehanort. The game appeals to the same people who are or would be attracted to the other KH games.

All KH games do this {I don't know about coded or 358/2} . BBS also focuses on Xenahort, Specifically how he gets his plan started. 
Xehanort's plan wasn't mentioned at the end of KH2 btw, as far as I know. You may be thinking of Coded. But even there the only thing mentioned was that Master Xehanort will return. Nothing about what his plan entails, iirc.  And learning how things came to be the way they are can be just as important as plot progression, although there is some of that in KH 3D as well. For example, the concept of the 13 vs 7 is introduced, Xehanort's plan is unveiled, the mark of mastery exam begins (and is compleated by one person), and Axel "returning from the dead" and learned how to weild a keyblade.

In KH 2 FM they reveal that Xenahort is coming back, and that they have to take the mark of mastery

And you can find common connections between DDD and the numerical games as well.

That's true for all KH games, BBS, COM, and DDD are the most closely related.
But there are two things I want to point out on your list.
Riku being a main character is not something indicative of a spinoff, whether it happened in COM or not. He has been one of the main protagonists since the beginning.

Alone it is not, but Riku is the main character of DDS, and I didn't say because you play Riku its a Spinoff, just that its a parallel that ties it to COM and in some ways BBS, more than the numbered series.
As for the battle system of DDD, the deck system stopped being experimental after COM. In BBS it was very refined. DDD's battle system has something from KH2 as well, namely the Link System. Which derived from the Dive Form system. And Kingdom Hearts 3 has the wall diving from DDD. It's not uncommon for Nomura to mix systems from different games, add new systems, or change old ones, to various degrees.

The Deck system command system and the card system are distinct from each other.  They work entirely differently. In fact, DDS has the card system as a minigame for dream eaters and its clear they are different there. The command system features the general combat of the numbered series, but turns skills and magic into commands that each reload on their own, this is in contrast to cards which are required for every action aside from movement. The Link system is a combination of BBS D-Links and Command Styles. In fact, Riku actually has some of Aqua's Command Styles namely Ghost Drive, and Sora has Ventus such as Salvation. Unlike Drives, Links are invoked by command usage, exactly like command styles. Drives are explicitly activated.

* story wise Drive's are fusions like Links, but Links are fusions that require no item, hence they can be learned at any time, unlike the Drives which require the Earrings, in fact, Drives are entirely tied to story, Final Form and Antiform are the only exceptions. 

Regarding numbering games in a series, one factor that can play in is marketing. If a sequel ends up on a different platform, or in a new market, the numbering the game may change, or be removed all together, if they think that will appeal to more people. It doesn't always happen, but it has happened many times. Metal Gear Solid: Peace Walker, which ended up on the PSP, might as well have been labeled MG5, but it wasn't. And many felt that Final Fantasy 11 should have been called Final Fantasy Online, but Square no doubt chose that name for marketing purposes even though the focus of the game is vastly different from the numerical series. And Shadow Hearts !! was renamed to Shadow Hearts: Covenant in the west, because they didn't feel like the series was recognizable enough in the west, and the number 2 might put people off from playing it if they hadn't played Shadow Hearts (1) first.

So this is something to consider as well when it comes to the naming of Dream Drop Distance, as well as Birth By Sleep.

No, it really is not, Square Enix and Testuya Nomura are very specific about their naming conventions regarding Kingdom Hearts, that's why they tend to be ridiculous and confusing. Dream Drop Distance is the exploration of the realm of dreams. Like COM it concerns an area that won't be covered in the next game, for COM it was Castle Oblivion. Also the Chain of Memories that tied Sora to his Friends, which was broken by Namine, so that Xemnas could create Xion from those Memories. Xion is a data clone like Dark Riku in COM.

And the term spinoff is not really applicable, because they are all relevant to the main story so side story is more applicable. However, Like COM and BBS and 358/2, DDD covers only one event, unlike the numbered series which cover an entire story arc.  

KH spin offs - are not spinoffs in the conventional sense, it makes no sense to compare them to other spinoffs. Every KH game is tied directly to the main story, not based off of it as in the case of most spinoffs. Its a really disengenous term, they just aren't numbered, since they do not progress the story so to speak.

All this is what I mean when I say DDD is KH2's COM