Yes. Sadly, DexNav appears to be an ORAS exclusive feature.
Necrozma with a form that turns into some kind of prismatic dragon thing? PLS PLS that would be so coooooolllll
It would be pretty cool and I'd love to see its story expanded. I mean, how does it tie into Solgaleo and Lunala if its meant to be a part of that trio.
I'm not really a huge fan of Necrozma's design, but this is the part I'm interested in. Because really, when you think about it, they're not a "trio" per se. Solgaleo and Lunala are just two diverging paths of the same base Pokémon, Cosmog. So in a sense, what we have is a duo - Cosmog and Necrozma. Cosmog absorbs light (in order to evolve), while Necrozma refracts it. Cosmog is only hypothesized to be an Ultra Beast, and Necrozma is only said to be "reminiscent" of them, and neither of them are registered as UBs by the Beast Balls. But the Cosmog line does have the ability to create Ultra Wormholes, and they're both said to come from another world, regardless.
So Zygarde, who has a blatant connection to Xerneas/Yveltal shouldn't be elaborated on but Necrozma, a legendary with no explicit connection to Solgaleo/Lunala, should? Seems legit. The hypocrisy is real with this game.
Again, Zygarde's issue isn't just a Zygarde issue. It's a trio-wide issue. Xerneas and Yveltal could be wiped from XY entirely and basically nothing would change. Xerneas and Yveltal are irrelevant, and consequently, Zygarde is too. (And I have said before that it would probably work better as a standalone Legendary like Heatran or Victini - a lone representative of a broad concept.)
Groudon and Kyogre are very relevant. Dialga and Palkia... I guess? I feel that it would make more sense for Cyrus to simply summon Giratina itself to cause annihilation. Platinum's plot could have worked without Dialga and Palkia.
Eh, not really. Ultimately, Solgaleo/Lunala end up being a glorified key to Ultra Space, they don't really have a lot of relevance themselves. The story is more about Lusamine/Lillie/Gladion's family and the Ultra Beasts than Solgaleo/Lunala, if you got rid of them the only thing that would change is how you end up getting to Lusamine in Ultra Space.
Groudon and Kyogre are pretty important to their stories. Nature is a big thing in the Hoenn games, and they represent the possibility of nature being thrown out of balance. They were also the original example of "ultra-powerful Legendary Pokémon that are sought by the evil team," so it was a novel idea for the games at that time. Of course, Rayquaza was basically useless in Emerald. They wouldn't end up doing anything interesting with it until ORAS. In Emerald, it's just a deus ex machina, and it doesn't even incite much of a revision of the lore about Groudon and Kyogre - what we basically get is Wallace saying, "Oh yeah, and in case ya' didn't knooooooow, there's a third super-ancient Pokémon that calms them down, even though we previously said that they were calmed by the Red and Blue Orbs."
Except Nebby being the key to Ultra Space is far, far more deeply rooted in the plot than Xerneas/Yveltal being a battery for the weapon was. Frankly, the weapon already had like, three methods of activation - the key, the power plant electricity, and the life energy of the Pokémon attached to the menhirs. Xerneas/Yveltal couldn't be more redundant. Nebby, on the other hand, is the only way for people to enter Ultra Space (or the only reliable way anyhow; they can't just sit around waiting for one to pop up at random). Therefore, Lusamine has a reason to want it, and Lillie has a reason to want to protect it. This affects the character motivations in a way that Xerneas/Yveltal simply do not.
It's not really deeply rooted in the storyline if it can easily be swapped out for something else though, and Nebby could be. Literally, you could replace Nebby with a turkey sandwich and it wouldn't really change things that much, Lusamine sure doesn't care whether it's Nebby or a turkey sandwich that opens the portal to Ultra Space, she just wants the portal open, and Lillie just wants to keep the means to opening the Ultra Space out of Lusamine's hands specifically because she doesn't care. Solgaleo/Lunala's connection to Ultra Space isn't inherent to their identity, literally anything can serve that purpose.
You literally can't say that until it actually happens. They're the ones who write the storyline, so they can add whatever elements to their lore that they want. Plus, just because something has more mystery behind it doesn't mean that they'll solve all of them in a satisfying way, they could easily end up leaving in plot holes, failing to connect elements in the storyline, or doing so in a convoluted way. Again, this all comes down to how clever the writers are. And Pokemon's writing isn't exactly Shakespeare level, so they could easily mess this up.
I think you're ignoring the intentionality behind using Nebby in that role though. Nebby wasn't just the power source, it was the mechanism. And later Solgaleo/Lunala are the only creatures we see capable of opening an Ultra Wormhole. There's clearly meant to be a connection between them and Ultra Space, it's not just a "oh by the way they can do this" sort of deal. And then we have a legendary who also appears to have some connection with Ultra Beasts. After Game Freak proved they can in fact tell a coherent and engaging story with legitimate twists, I would be shocked if they didn't follow up on the three of them in a future game. I don't think we can use previous game traditions and tendencies to determine exactly what this role will be, because Sun and Moon broke so many of those already especially when it comes to plot.
Sorry, but no. Lillie wants to keep Cosmog out of Lusamine's hands because it's a living creature and Lusamine doesn't care that it has to die in order to open the portal. Lillie isn't principally concerned with merely keeping Lusamine from going to Ultra Space, because that in itself doesn't harm anybody. If it only required some inanimate McGuffin, the storyline would have to be rewritten. Lillie would lose the parallel to Gladion stealing Type: Null for similar empathetic reasons, and she'd have to object to Lusamine's goal for different reasons rather than the protection of a living being, and the scene where we save Nebby from the Spearow would be out ("Oh no! I dropped my Wormhole Key on the bridge and those Spearow are going to mercilessly fly away with it!"), which subsequently necessitates a change in the circumstances of our encounter with Tapu Koko.
Speaking in terms of general principle here. I have an easier time letting something like Zygarde's situation slide because the dynamic is exceedingly easy to infer anyway. Whereas a less-clearly-associated Pokémon could be any number of things, and there's just no way to know until they give us more information to work with.
I didn't say that any of this wasn't a possibility.
It could be that, but as you say, in most previous cases they gave us plenty of clues to work with. Necrozma's sitting at about where Giratina was - we know it comes from a parallel dimension and shares traits with other important Pokémon, but people had lots of guesses about how Giratina pertained to Dialga and Palkia exactly. It representing antimatter actually was somewhat unexpected, if I recall. I seem to remember it representing death was one of the leading theories.