Latios and Latias
Eggs should have hatched as of 30 minutes ago; Lugia is gone and the Eggstravaganza is also over.
Latios: North America, South America, and oddly, Africa
Latias: Europe and Asia
They switch on
May 8 and leave
June 5. Plenty of time to dedicate to them. There is also a Raid Boss shakeup in order to counter the Eon Duo, so expect the usual culprits like Gengar and Tyranitar to still be Raid Bosses, and some added new ones to the mix.
The best counters to both of these are Dragon-types, such as Rayquaza, Dragonite, Salamence, and Latios itself. They have relatively weak movesets for Raid Bosses (except Solar Beam on Latios) so Ray and co. can handle the DB / DC movesets as well. The other efficient counter is Tyranitar but it must be wary of Solar Beam from Latios. More budget counters include the Ice-types (like Jynx), Scizor, and Houndoom.
Metagame relevance: Little. Latios is a fantastic gym sweeper with Dragon Breath / Dragon Claw, but this isn't as lucrative in today's metagame and DPS-wise it's behind Rayquaza and Dragonite. Latias is an inferior Latios, of course.