What's next (sharing one day with Latias)?
Question says it all. Suspect most of us want Dialga, think we might get Regigigas or Origin Forme Giratina, and are worried that we'll get the lake trio. Anything else?
Of course, two weeks of Rayquaza is a possibility, albeit a bit of a stretch given that we've now all gotten lotsa Mamoswine to shred it with. I suspect a duo is easy, but that a solo is not in the cards, even with weather and the ideal moveset. Or two weeks of some other retread, packing a shiny form.
Let the idle speculation begin.
Answers
One other possibility that I just realized. It's possible that Palkia departs today at 1 PM PST, leaving just Latias as the raid boss for a day, and then tomorrow at 1 PM PST, Latias leaves and something new replaces it then - so Latias is acting as the stopgap legendary for a day. So all of this 'what's next' hype (that I freely confess to helping drive) might be deferred a day.
My guess is Latios and Dialga will be dropping next, with Latios lasting 10 days (ish) but Dialga being here all of March.
April I expect either Origin Forme Giratina or Regigigas, then May will be Rayquaza. Kind of a "Kick off the Summer" idea. Then in the summer, I expect Uxie, Mesprit, Azelf, and Darkrai to be Raid Bosses (with the Lake Trio likely having a month for all 3, not one month each; I think Niantic has learned from the Regi trio last summer that useless Dex fillers are not attractive to Raiders), with Arceus, Shaymin, Manaphy and Phione being available via this summers Research Quest (like Mew, Meltan, Celebi, etc.).
(Though given how powerful Shaymin Sky Forme is, I could see that being a Raid Boss instead of a Research Quest as a hell of a lot of us might want to Raid to have 6+ of them rather than cross our fingers that our ONE Shaymin Sky Forme has good IV's...)
I'm expecting that the Arceus Quest is going to take months to complete. I wouldn't be surprised if it included things like "Catch the base form of 20 different Kanto/Sinnoh/Johto/Hoenn Region Pokémon", "Evolve the Stage 1 form of 20 different Kanto/Sinnoh/Johto/Hoenn Region Pokémon", "Evolve the Stage 2 form of 20 different Kanto/Sinnoh/Johto/Hoenn Region Pokémon", to "Spin 100 New Pokestops", and almost certain to include "Catch 4 Mythical Pokémon", etc.
As for Jirachi? I'm expecting that we're going to get that this year. Probably a week-long event, either beginning or ending with Tanabata.
(or, you know, Shiny forms of the Regis are released)
I'm expecting Arceus to be a quest that will take months to complete, and that will be designed to make us excited to see meta-irrelevant Pokémon (like Sandslash, or Chinchou) because they're now vital to completing this quest.
So yeah. I'm expecting the quest to look totally whack when most players want what amounts to instant gratification. (Remember how loudly people complained about the "Spin a Pokestop for 7 straight days" was a Quest?) I'm expecting Niantic to make the Arceus quest something that you're going to have to work at, and commit a great deal of time to.
The other ones, probably going to be able to be done in a day or two of heavy grinding. But I'm expecting Arceus to be a long quest.
This isn't about instant gratification, this is about wanting a research line that's actually possible to complete.
Spin 100 Pokestops you haven't visited before? Remote players had a hard enough time meeting the 8 stops needed for Spiritomb, you think they're gonna upset that portion of the playerbase to the point of near riots? Hell, I'm a city player and it would take me an impossible amount of time to complete this without day trips to neighboring cities. Catch 4 mythical Pokemon? Pray tell how someone without a Switch would complete this, or someone who's already completed the other special research tasks. People complained about "Spin a Pokestop 7 days in a row" because it became an unnecessary time gate for what's supposed to be a "play at your own pace"element of the game - that and it was bugged initially, preventing people from completing it when they were supposed to.
Honestly, if someone wants to bang out the special research in a couple of days why shouldn't they be able to? That's their pace, and likely means more profit for Niantic via tasks that require raid completion. Niantic has stated recently they felt the special research we've already had were unforgiving to some players based on community feedback, do you really expect them to ramp it up to 11?
One point I agree on, I'd actually be in favor of "catch [X] amount of [Y Pokemon]" that have incredibly lofty numbers. That's the truest you can get to playing at your own pace without being interrupted by time gates or paywalls.
For Arceus? Yeah, I do expect them to ramp things up - to go for something long-term. They're going to be dropping the Wizarding World game in 2019, so they're going to want to have something to keep PoGo players interested and committed to playing in the long term. They're not going to want player to go "Nothing more for me to do here, why not try this new game?" and drop PoGo - so having a long term, substantial quest for the Original Pokémon is how I'm expecting them to do that.
Maybe Spin 100 New Pokestops is excessive, but it's just a number. 50 New Pokestops, 75 New Pokestops, 40, whatever - a significant number that makes it a significant task to complete. And at this point, that's not going to be an issue for just Rural Players - I probably couldn't find 100 New Pokestops in my city to spin either unless they've gone on another spree and added new ones. I'd probably have to go somewhere else to get that done. This wouldn't make the quest impossible, nor even improbable. (something like "Complete all the Pokedexes!" would likely be impossible due to all the Regionals that are stupidly expensive to trade for, or "Have one of each Legendary!" or the like.)
I already answered how to catch 4 Mythicals - Shaymin, Manaphy, Phione and Jirachi all being dropped with "regular" Research Quest timelines.
Quibble about the examples, and even if I'm right about them setting up Arceus for a long term quest, I'm likely going to be way off about the exact quests. But I'm expecting Arceus to be a quest that will take even the most dedicated Pokémon Go player months to complete as a way to keep players motivated to play Pokémon Go while Wizarding World is released.
My guess on Arceus is that it won't be that hard, because they will want us to be able to do the quest more than once, for the different forms of Arceus.
If it were up to me, I'd have a different kind of quest that you spin from a stop that when finished, opens up a special research quest as a 'reward' for completing that quest; the SR quest has several steps and will take hardcore players a few days and casual players maybe a month, and will reward a specific typing of Arceus. Then, later, you'll spin a different stop and get a reward of a different Arceus SR quest. Probably have stuff in place so that you can only be doing one SR Arceus quest at a time, so that it takes much time to collect them all (and that's the point, per the motto, right?)
Maybe something similar to that for Shaymin, but with one for each form, and the quests get harder if you want to get a second one, etc.
