Two years ago, when Bethesda revealed Fallout 4 at E3, the company didn't want us to have to wait until November to play some Fallout. So they announced Fallout Shelter for iOS and Android. The game became a mega success for Bethesda, leading to a new game studio being opened in Montreal.
This new studio has not working solely on mobile games, but also on PC titles as well. It's been two years since the studio opened its doors -- and therefore we could very well see the first fruits of its labor. Bethesda made a point of saying that there is no plan in place release a mobile game alongside every major AAA release they put out, but that doesn't mean there aren't any mobile games at all coming in the next few years.
I'm not sure what the next mobile game from Bethesda will be. The obvious option is another Elder Scrolls tie-in -- but I think not, especially considering that Elder Scrolls Legends just released. But maybe we'll see something akin to Hitman Go in the Dishonored universe?