Although Pokemon GO's Raid Battles haven't been around for too long, fans of the mobile release are already wondering which creatures will be added as Raid Bosses after the 20 different ones that are currently in the title are cycled out of the augmented reality game. As it so happens, the ARG's website points to four different Pokemon possibly being incorporated as Raid Bosses sometime down the line.
Should one look on Pokemon GO's official website, they will initially find that the Pokemon Dragonite is the most likely candidate to become a Raid Boss, as Niantic has used the creature in several different images to explain the Raid Battle's mechanics. Other than the Dragon/Flying-type, other creatures that have been linked to Raid Battles through promotional pictures are Blissey, Ninetales, and Charmeleon, as their inclusion as Raid Bosses in the future is also likely.
As far as the aforementioned Pokemon's credibility as Raid Bosses is concerned, with Dragonite being the Gen 1 equivalent to Tyranitar – that is, the most difficult Raid Boss Pokemon GO players can face so far – it would make sense for the former to supplant the position of the latter in later updates. Not to mention, Blissey has a high HP much like Snorlax does, so it will need a large group of trainers to take it down, making it a great selection for a Raid activity.
When it comes to Charmeleon, players have already encountered Quilava, Bayleef, and Croconaw as low level Raid Bosses, so it's quite likely they will be cycled out as starters for Charmeleon to eventually take one of their places. Not to mention, Ninetails is currently in a similar position with Arcanine already being a Raid Boss in Pokemon GO.
All things considered, since Niantic hasn't officially confirmed these as additions to Pokemon GO as Raid Bosses, there's a chance that none of them make their way into the game. Nevertheless, with the developer having featured them all in some capacity to promote Raid Battles, their eventual inclusion as Raid Bosses is much more likely than any other creatures that can currently be found in the game.
Pokemon GO is available now for Android and iOS mobile devices.