Given that the only text in the tweet is #PleaseStandBy, we can only speculate at this point as to what this teaser means. The most obvious answer could be that Bethesda is planning to port Fallout 4 to the Nintendo Switch. Fallout 4 was released two and a half years ago for the PC, Xbox One and the PlayStation 4.
And given that Bethesda has already released DOOM (surprisingly) and Skyrim to the Nintendo Switch, it wouldn't be tooterribly surprising to see the game appear albeit with compromises made to visual fidelity.
Another possibility is that Bethesda could be looking to offer a remastered version of Fallout 3 for modern gaming platforms. Fallout 3 was released on October 28th, 2008, so it's almost time for the 10-year-anniversary. We know that developers like to cash in on the remastered "gravy train", so this is also a probable outcome.
Or this could be something entirely new, maybe along the lines of Fallout Shelter. A job listing posted around this time last year suggested that Bethesda is working on a "bleeding-edge AAA freemium game". Perhaps that is what we'll see announced in the coming weeks.