6e/50th Anniversary Edition in 2024.
Not sure whether you're joking or not, but I think we'll see something in 2024. A 50th edition only comes around once and it's a pretty big deal. So, I'm going to say that 6E will come out, then.
5E can certainly be maintained that long. IIRC, that'd be about the same run as 1E and that's close to the release cadence we've got, right now. We got a monster book, last year. This year, we're getting some crunch. We could probably do two more monster books and one or two more crunch books without any risk of bloat or fatigue. Assuming 2023 is some sort of filler year, that only leaves two years without a major release (yeah, that's shorter than I thought). I could see those either being bigger years for promoting adventures or (hopefully) something SCAG-like for another setting.
Here's what that could look like:
- 2016 -- MM2 (Volo's Guide - included to establish numbering shorthand)
- 2017 -- PHB2 (Midway)
- 2018 -- Planescape (because this seems to be where most speculation is about the next setting)
- 2019 -- MM3
- 2020 -- PHB3
- 2021 -- Eberron (because I like it)
- 2022 -- MM4
- 2023 -- Greyhawk crunchless setting book (because it sounds like something that would be a cool lead-in to a 50th anniversary, GH really doesn't need any crunch, and people might buy it after 6E is announced)
- 2024 -- 6E PHB
Now, my hope is that 6E is as compatible with 5E as 2E was with 1E. That's not necessarily because I plan to keep using all my 5E stuff instead of buying 6E. It's because, while I'm sure there's room for improvement in 5E, I think they've got something that's actually pretty strong and stable. We don't need changes on the scale of AD&D to 3E or any later edition change.
One complication could be a "5E Revised Edition" that shaves off some of the rough edges. I suspect that the PHB2 and 3 will have options to "fix" somethings, but 2019 would be a good place at which to do a more formal update.