Fallout 3 and Fallout New Vegas both have updates pending according to SteamDB. These are not small updates that would fix bugs, since the one tied to Fallout 3 is 5,9GB in size and updates for both games are listed under the DLC tab.
Fallout 3 and Fallout New Vegas may be getting remaster treatment according to the info from SteamDB. Granted, this is our conclusion, the site simply provides information it owns, but the information is rather incriminating at the moment. For example, both games have been updated at the same time - on 08 May 2018 and the size of Fallout 3's update roughly matches the size a pile of new textures would have.
There is no file size tied to New Vegas' update which may hint that this game will be getting a remaster at a later point, if the theory turns out to be true. Another fact that is pointing towards remasters is that Fallout 3 will celebrate 10th anniversary this year which seems to be too good a cue to pass upon. Besides, it will be ported to Nintendo Switch and this version will be shown off during E3 2018. Porting an old game to Switch is the perfect time for a remaster, as it proved to be the case with Dark Souls.
The lack of size data in SteamDB's Fallout New Vegas folder likely points to the fact that the remaster progress isn't anywhere near that of Fallout 3. New Vegas is also nowhere to be seen in the torrent of leaks that flooded everyone and every news outlet prior to E3. It would also make no sense to release both remasters at the same time on Bethesda's side, since one could hamper the other's sales.
It is not excluded that New Vegas remaster is also almost done, and that it could be shown at E3 since Pete Hines mentioned that this E3 will be huge for Bethesda and that they still have many surprises left for the fans, despite the reveal of Fallout 76 and the leaks surrounding Fallout 3.