It seems that a release date is due to be announced soon for an upcoming Bethesda title, as a page has appeared on Amazon for an unknown game that is attributed to Bethesda.
The Amazon page lists three different editions of the game: Collector's, Deluxe, and Standard, all of which cost originally cost $59.99 (via Wario64 on Twitter), but these prices have since been taken down. It's possible to purchase the game for the PC, PlayStation 4, or Xbox One.
The release date for the game is listed as December 31st, but that's almost certainly a placeholder, as we've seen them used before in similar situations for games that have only have the year of their release date announced.
In terms of Bethesda games that have been announced but don't have a release date, the possible candidates could be Starfield, Wolfenstein III, Wolfenstein: Cyberpilot, or Wolfenstein: Youngblood. Doom Eternal might also have been a possibility if it weren't for the fact that it has also been announced for the Nintendo Switch and the Amazon page doesn't have a selection tab for that system.
The Elder Scrolls VI is also a possibility, but we have seen so little of the game that it's hard to imagine that it would be released so soon. The release of a new Elder Scrolls game is a huge event and it's something that is built up by years of promotion, yet we know nothing almost nothing about the game.
There is also the possibility that the placeholder is for a brand new game that Bethesda has yet to announce. GDC 2019 is only a few weeks away and Paradox Interactive is hyping up some kind of announcement during the event, so it's possible that Bethesda might be planning the same.
It's also possible that the placeholder game is for something that will be announced at E3, even if it's a bit premature to be adding placeholders so soon. It would also have to be a pretty huge announcement to win back the fans who turned on Bethesda after the disastrous launch of Fallout 76.