Bethesda's current focus goes like this: Starfield -> The Elder Scrolls VI -> Fallout 5
It's going to be a long, long time before another mainline Fallout releases from Bethesda Game Studios. In fact, the studio has about one page of plans for the sequel. Right now Bethesda is all-in on Starfield, their first major new IP in the last 25 years that will not only flex a brand new next-gen games engine, but establish data, content, and features that will power all future games from the studio.
Then after Starfield the team is going right into The Elder Scrolls VI. But what about Fallout? There's not much to say about a Fallout 5.
"Fallout is really part of our DNA here. We've worked with other people from time to time, I can't say what's going to happen. We have a one-pager on Fallout 5, on what we want to do," Bethesda Game Studios Game Director Todd Howard said in a recent IGN Unfiltered interview.
"Again if I could wave my hand and have that out...I would like to find a way to accelerate what we do but I can't really say today or commit to anything on what's gong to happen other than our cadence is Starfield and then Elder Scrolls VI."
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Howard also makes an interesting point regarding Fallout 76 and The Elder Scrolls Online.
These live games are designed to carry the franchise forward and bridge the gap between mainline sequel releases. It's been 10 years since a new Elder Scrolls, but ESO has done a great job carrying the IP forward.
The same is true for Fallout 76, which has carried the ongoing 6-year gap between Fallout 4 and the next Fallout.
So for the time being don't expect to see another big Fallout 5 sequel from BGS. Other partners could jump in and do something similar to Fallout New Vegas, but for now all we have is Fallout 76...for better or worse.