Lol, Ventus and Lauriam have the strongest likeliness to be killed off permanently becuase they're featured in Union Cross. Union Cross will continue on betond Kingdom Hearts III, therefore, Ventus and Lauriam can still exist in the Kingdom Hearts series whilst still being killed off in Kingdom Hearts III.
This shows again you completely misunderstood what Nomura is actually stating there.
He's speaking about "some" characters that
probably never appear ever again. Meaning after KH III they won't appear anywhere anymore.
Characters appearing and still being around in Union X do not fit into that category because they
do appear again.
For me, there are a few reasons why I would have liked to know if, at least, the Radiant Garden crew returned.
Of course, there's the whole "first people you meet on your journey" and they do act as a guide for Sora, and the player. I've appreciated their position in Kingdom Hearts 1 as the 'sages' guiding Sora through his new abilities, and offering themselves as the backboard to the story. If things got tough or confusing, you could rely on these people to help you out. They quickly moved from mentors to friends, all banded together with a common goal.
And it's the common goal part of these characters that I've enjoyed. Outside of Yen Sid and Mickey Mouse, Sora doesn't really have many people to look up to and work with on a common goal. The Final Fantasy characters offered the idea of other people out there in The Other Sky working on the same plight as Sora, that there was hope in the hearts of people living on. And it feels like that's been stripped away in KH3 just so Sora can be overwhelmed by villains.
Hmm, I see where you coming from. It has to be kept in mind though that Nomura by now has repeatedly said he cannot talk about things or characters who haven't been revealed yet so him giving dodgy answers to questions that don't concern the current "info-level" of trailers and magazine reveals is normal.
If the Radiant Garden crew returns will be decided in one of the following info dumps the next seven months, probably alongside more original character reveals.
I get that, but this "mentor"-role was largely confined to KH 1 and partly CoM as ever since SE and Nomura can use Mickey more liberally him, Yen Sid and to an extent Naminé have taken over those exposition/mentor roles. Even Ansem the Wise can count into this group too even if he isn't often personally present.
I'd assume that the "common goal" part is at least in KH III to be filled by the other original characters, most prominently Riku, Kairi and Lea but also Roxas, Naminé, Xion and the BBS trio.
The main difference this time around is that most of these allies aren't already gathered and helping Sora out as in KH 1, but that Sora has to help them out and gather them with Riku's, Kairi's, Lea's help
before the villains can become too overwhelming. Like a reversal of how KH 1 was set up.
It's also an opportunity for Sora to show how much he has learned since the times of KH 1.
There's of course also the possibility that Nomura is going the same route as he did in BBS, dropping the FF characters in favor of Ansem's apprentices/Former Organisation members like Ienzo, Aeleus etc. to portray their change of allegiances, which I personally wouldn't like too much myself.
And then, with Kingdom Hearts 2, these characters had fully moved from the mentor role and it felt like you were finally equals with Leon and Yuffie. I was invested in their plight to restore their world, and they provided some nice fanservice moments during the war. You also got random, out there cameos from Yuna, Rikku, and Paine (although it is questionable if they were even a good cameo lol) so the whole angle of "you never know who you're going to meet next" seemed to now apply to Final Fantasy characters. It's a shame that was dropped after this game.
With the end of Radiant Garden's storyline in KH2, there was hope for the return to the Capital of Light, to see the crew having restored their world into a bastion of peace, hope, and light. For a game about conclusions, it could have been the closing chapter on that world and this group of Final Fantasy characters.
Instead, the reluctance to show anyone makes me fear that Radiant Garden will not be seen at all, that we won't get the conclusion to their story in a game about the conclusion to a bunch of other stories. Sure, it's probably nostalgia for me to want to see these characters one last time and to close out their journey alongside Sora as peers, but it really feels like a lost opportunity to really have the world be another central hub with a bunch of relevant lore (Kairi's homework, Ansem the Wise's research, the birthplace of the Organization).
We know there's one cutscene there, as evident by the recent trailers, however the data leak from December also had Radiant Garden data in it but it only contained that room Ienzo was in. Also the restaurant in Twilight Town that you cook with Remy in is owned by Scrooge McDuck (more on that coming soon) has me thinking he won't be in Radiant Garden any more, so I'm a bit worried that Radiant Garden will completely miss out on being a playable world.
On the other hand, Nomura says there's a bunch of original worlds they haven't shown off so I'll have that tiny bit of hope in my heard for the world to fully return as playable, with these characters that are friends.
Since these are the most involved FF cameos I would say that if Nomura doesn't choose to omit FF characters completely, Leon & the gang have the highest chance to be featured out of all available candidates.
Yuna, Rikku and Paine on the other hand I can easily seen being dropped completely while i.e. Cifer & co. could be just regular NPCs running around in Twilight Town without a single cutscene involving them.
Honestly, I'm one of those who loves the atmosphere and design of the original Radiant Garden seen in BBS and I'd love to explore a world restored like that in the new graphics engine, so RG being excluded would be a quite a bummer for me too.
Like I said above, the "reluctance" to show anyone or say anything definite may be because they simply can't talk about stuff that hasn't been shown yet and it may be among the worlds not so far in development yet.
Radiant Garden is one of, if not
the biggest of the original worlds with a huge city, a castle, gardens, water features and much more stuff in it that would take quite some time to create in the new engine, especially if they want to also add new areas.
It's probably a similar case as Big Hero 6 in terms of scope and thus a sort of challenge for the developers and designers just because of the scale.
As for Scrooge, in BBS he did say that he was intent on creating a cross-worlds spanning business empire so it is not surprising he might be in Twilight Town for it this time around.
In order to not make RG too overcrowded with the ex-Org members and possibly Leon's gang around some characters formerly in RG could also be moved to other original worlds.
And as you say, there is much to be resolved in RG that also concerns many of the original characters, specifically Kairi, Ansem the Wise and to an extent Aqua (due to the Chamber of Repose) so I'm somewhat hopeful that it still will appear in some manner.
Thank you for such a detailed answer too, very much appreciated.