Been playing catch up reading this thread. Some questions.
1) Is the
serebii list of confirmed Pokemon currently the most up to date? Likewise for their list of confirmed new/returning megas.
Going to answer this question and springboard into some of my own thoughts.
For a while it wasn't, but it seems to be up to date now. IGN's most recent preview last week revealed that Cleffa was in the game, and that's finally been added in, so I assume that it's up to date with everything that's been shown off in official materials.
Since Mega Evolutions are a necessary mechanical component for teams to win Rogue Mega Evolution battles, I'm intuiting that the dex is overwhelmingly going to be Pokemon that have Mega Evolutions in their families. It wouldn't make sense to distribute too many options that have to be benched for repeated challenges - imagine if you could only bring a small pool of Pokemon to max raids.
From there, I'm predicting that Legends ZA will follow Arceus's lead of having all newly introduced Pokemon (and their families) from the generation it's "remaking", so I wouldn't be surprised to see Gen 6's fossil Pokemon, Carbink, Xerneas, and Yveltal pop up in the game even though they haven't been shown in pre-release materials.
There's also a handful of Pokemon that I think are picked for the game for different reasons. Pikachu is always going to be an easy addition, and the DLC Mega Raichus further justify its place in the base-game. Then, you have Pokemon that just haven't been available in a core series game since 2017. Watchog and the Simis are the only Pokemon that fall into this category, so it makes sense that they'd get squeezed in the last Switch title.
With what I've stated above about Mega Evolutions being mandatory to progress through certain challenges, the Pokemon revealed in pre-release materials that lack Mega Evolutions seem to be the most likely candidates for getting new Mega Evolutions. Even though Generation 6 introduced starter Pokemon and didn't give them Mega Evolutions in their debut, I think it would make more sense for Feraligatr and company to get some so that you aren't locked out of using your starter during climactic moments.
Trailers have shown that there's several story-relevant and named NPCs that can use Mega Evolution, so these are pretty good hints that the Pokemon they're seen using (Staryu, Scraggy) are pretty poised to get new Mega Evolutions in their families. Maybe there's some hope then for ones like Arbok, Clefable, Ariados, Roserade, Hippowdon, the Simis, Watchog, Krookodile, Garbodor, Emolga, and Chandelure. Getting a new form between Gen 7-9 isn't exclusive from having a Mega Evolution as seen in the case of the Generation 1 starters, Pikachu, Eevee, and Slowbro, but I think GMaxxers like Machamp and Garbodor will probably get skipped for the Mega treatment. Slowking and Froslass could also be up in the air with one member of their family having a Mega. With the confirmation that a base-game Pokemon like Raichu would be getting a Mega Evolution, it's also not out of the question that more of these Pokemon will get DLC Megas.
One thing I haven't seen much discussion of is how Rotom is in the game as your phone... but it hasn't been shown in the wild! XY did have Rotom available in its base game, and I think it would be pretty funny to have Rotom as part of your Dex without having it be catchable.
Some of the things I'm less certain about are legendary and mythical Pokemon. Legends Arceus was very generous with distributing all of the Generation 4 legendary and mythical Pokemon, and it threw in a few more just for good measure. ZA's focus on Mega Evolutions and remaking parts of Generation 6 probably indicates that Diancie, Mewtwo, the Latis, Rayquaza, and related Pokemon will be in the game. I would be surprised if Mew didn't appear then in a game Mewtwo is pretty likely to be in, and the same goes for Kyogre and Groudon wrt Rayquaza. I wouldn't be surprised if some of the ORAS content gets incorporated into the Mega Dimensions DLC, especially considering that Hoopa is set to be prominent in that. Volcanion is the oddest one out of anything, but its inclusion as post-game content would make a lot of sense given the way Legends Arceus handled Manaphy, Darkrai, and Shaymin.
XY highlighted a handful of Pokemon in its experience that have yet to be featured in pre-release materials for ZA. Rhyhorn and Mamoswine were both used for exploration in sections of XY, while Snorlax and Lapras were available to the player in echoes of their encounters in RBY. These lines tend to be pretty popular, so they might still show up in the game.
Conveniently, the amount of revealed Pokemon, Pokemon in families with a revealed Mega Evolution, and Generation 6 Pokemon totals up to around 250ish, and Legends Arceus had 242 Pokemon in its game. I'm anticipating a pretty similar dex size, so I would say we've seen about 80% of what will be available in the game, and 90% of what will be available is pretty easy to predict.
The biggest things that I'm excited for are the surprises. Legends Arceus followed in the footsteps of Pokemon always iterating and breaking "rules". Thundurus, Tornadus, and Landorus showing up in Hisui and then introducing Enamorus and its own Therian form was pretty unexpected and a neat way to elaborate on SWSH's DLC adding Regieleki and Regidrago. Hisuian Goodra, Zoroark, the Hisuian regional forms and evolutions also fall into this category, but I think that ZA will mostly go the way of emphasizing Mega Evolutions since, like I keep repeating, they're critical to the game's challenges. I still think there's unknown unknowns though that will be fun surprises, like how Alolan Ninetales was hanging around Hisui with its regular form.
