Pure speculation, but there is valid info to support the hypothesis.
The next BGS game will be Starfield. It has been in development for 4+ years if we go by the trademark filing records. It will have a new game engine supporting it, as Mr Hines is on record for stating that BGS had planned to continue using the Gamebryo/ Creation Engine foundation for one more title after Skyrim. (That would be FO-4.)
Summing it up:
New IP - likely Starfield - One Project
New Game Engine to support off planet exploration dynamics such as space flight and space weaponry use, possibly even some procedural generation - 2nd project
New mobile game- paired with new IP ala FO -4 3rd project
After this I think ES-6 is almost certatin to appear probably 2019-2020... Fallout - Insert theme here will probably fit in there somewhere also, but not necessarily be totally produced by BGS. (The Battlecry Studio game has dropped from promotions, so what is the studio devs up to?) Also would be very surprised if Chris Avelone wasn't contracted to write the story for it, he is currently contracted for another Bethesda/Zenimax titles....
Footnote:
Do find it interesting that Pete Hines tweeted / confirmed earlier this week the Bethesda would be doing their 3rd full blown press conference at E3. Now IGN writes an addendum type piece with Todd Howard updating the BGS project roster, if only marginally. I don't think it a coincidence, seeds are being planted for us to take interest, and begin a bit of hype flow...