Really? They're implementing a pseudo-roaming mechanic for the Legendary Beasts? Oh, that should be fun, your continent waiting for its turn. At least it's for the entire continent and not specific locations (though why not have each one appear in a different continent instead of all the same)?
Now that makes me wonder what they'll do for other Legendaries with puzzle elements:
Titans (can we please settle on the name for these guys?) + Regigigas: You're alerted one is nearby and you have to do the actions the games have you do in the chamber to do the encounter. You have to walk zigzags to get Regirock, have to wait in one spot for two minutes for Regice, and walk in a circle a few times for Registeel. Than once you got all three you gotta encounter another Regice but just interacting with it will have it be a Regigigas.
Eon Duo: Half the players can only get Latios and half can only get Latias. In order to be able to encounter the other you'll need to battle the other from a Gym (or one-v-one battles if they ever introduce them). Your next Eon encounter will then be the other one, but after that it returns to being the one you default to.
Weather Trio: Can only catch Groudon on hot days, Kyogre AFTER rainy days, and after one of those days the next windy day will be a Rayquaza.
Deoxys (it was declassified as a Mythical): After a windy, rainy, or snowy day, the next nice night after the day it cleared will have Deoxys appear.
Lake Guardians: Only appear near PokeStops that represent their spirit: Uxie near libraries and museums, Mesprit near theaters and hospitals, and Azelf near monuments.
Creation Trio: Everyday Dialga & Palkia will randomly appear in random locations for one hour. The next day in a few of those spots a Giratina will appear.
Lunar Duo: Cresselia only catachable during full moon, Darkrai only during a new moon.
Heatran: During hot days may appear around a monument.
Swords of Justice: Eh, same as the Legendary Beasts. They don't really have a capturing gimmick.
Forces of Nature: Tornadus on a windy day, Thundurus AFTER a lightning storm, and after one of those days the next sunny day will be a Landorus.
Reshiram & Zekrom: Same thing as Eon Duo, though instead of only one encounter you switch over to encountering the other one all the time.
Kyurem: During Winter for a day AFTER a day of snowfall.
Xerneas & Yveltal: Xerneas during nice days in Spring & Summer, Yveltal during a windy day in Autumn & Winter.
Zygarde: After every Legendary encounter, there's a chance a Zygarde Cell or Core will appear. If its a Cell you just collect it like an item. If its a Core, depending on how many Cells you have can turn into 10% (if you have 10), 50% (if you have 50), or Complete (if you have 100). Note after each encounter you lose the Cells you collected. Maybe they could also introduce a way to change the ones you caught between forms via turning it back into a Core and giving it a number of Cells.
Guardian Deities: Eh, same as the Legendary Beasts. Only thing I can think of would involve Z-Move poses which I guess they can do with a camera, but nah.
Cosmog Line: Appears during cloudy days, some players more likely to find it during the day and others at night. What time of day you caught it will determine what it'll involve into: Solgaleo for daytime or Lunala for night. But that can be changed by having Cosmoem battle the other evolution, changing it to evolve into that one.
Necrozma: I say whatever they do for the Ultra Beasts would have Necrozma connected to that, like it may appear a day after in a location of an Ultra Beast encounter.