This comes as LPVG also stated that Dark Souls 3 is currently running on the Nintendo Switch as From Software is seemingly preparing to release Dark Souls 3 and all its DLC content on all platforms on the same day. Ashes of Ariandel was released on October 25th and a second DLC is planned for release.
From Software was already shown to be one of the Nintendo Switch partners, released by Nintendo themselves:
It seems that the release of the trilogy would be dependent on the reception and sales of the Dark Souls 3 collected edition. Dark Souls 2 and 3 would be easy ports as From Software could choose between their PC or Xbox One/PS4 builds, but it gets trickier when it comes to the original Dark Souls game.
Originally released for both the Xbox 360 and PS3 on 2011, Dark Souls was a critical success but full of bugs. The PC version was also riddled with bugs, which makes it unlikely that From Software would use that build if it were to port it as part of a Trilogy to the Nintendo Switch or other systems.
A remaster of the original Dark Souls would be likelier, so that both visuals and performance are more on-par with the two sequels. Which is why it makes sense that they would test the market first with Dark Souls 3 and its DLC releasing first together. At least, this should be like Mass Effect 3 releasing on the Wii U and being more expensive than the Trilogy which had already come out for both PS3 and Xbox 360.
Namco Bandai games is the publisher of the Dark Souls games and have a good relationship with Nintendo, having worked on games like Tekken Tag Tournament 2, Pokken Tournament and Super Smash Bros for the Wii U.
The possibility of taking the Dark Souls trilogy with you on the go, and being able to team up with a friend via local multiplayer without needing to connect to the internet is something worth considering and almost too good to be true. As far as Dark Souls potentially being a portable game? Praise the Sun!