The Bethesda Game Studios Twitter has been updated with a cryptic / not cryptic image of the "Please Stand By" loading screen typical of Fallout games along with the hashtag #PleaseStandBy. This leads many to believe that a Fallout announcement will be made at this year's E3 and that it could mean an expansion for Fallout 4 or something really exciting like Fallout 5 or even a Fallout MMO or RPG.
PCGamesN speculates that it is a brand new game: "The logo itself tells us a little more. It’s a different color to that of Fallout 3 (green), Fallout New Vegas (orange), or Fallout 4 (grey)". Take note of the larger amount of purple and you can get the gist of the idea.
Because the post came via the Bethesda Game Studios account rather than the Bethesda publishing arm, it's likely that this is an in-house project as well.
What do you think it could be?