Pokémon games launch in pairs, but through the first four generations, Game Freak also released a third, enhanced version at a later point. According to several sources, that will be the case with Sun and Moon as well, but there's a bit of a twist. Rather than launching on 3DS, Eurogamer reports that a third version of Sun and Moon will launch on Nintendo Switch.
Eurogamer claims that Pokémon Stars has been in development alongside Sun and Moon, but its progress was halted over the last couple of months so that the development team could put the finishing touches on the 3DS titles. Now that they're out in the wild, the team is back to working on Stars, and it's planned to launch sometime in 2017. Eurogamer believes it will be released sometime after the Summer.
Stars will feature the same game world and locations as Sun and Moon, but it's built using separate, higher-resolution assets. Stars will have features that aren't included in Sun and Moon, and Eurogamer has even heard rumblings that it will contain more Pokémon as well, but they haven't been able to confirm that last bit.
This information is not officially confirmed by Nintendo at this time (and I'm sure they're not pleased about it surfacing), but Eurogamer has proven highly accurate regarding Nintendo Switch leaks. They were the first to report on its use of detachable controllers and cartridges months before it was unveiled. Additionally, this report has been corroborated by Laura Dale of Let's Play Video Games (who has also proven accurate regarding Switch leaks), as well as insiders Emily Rogers and Liam Robertson.