I've shared my thoughts in other threads, but it is a fun topic and I enjoy speculating, so...
First, re:
modular options. Frankly, I don't think they'll ever deliver on the initial Next concepts (remember the "
complexity dial?"). We'll see more options and sub-systems (e.g. psionics) but the complexity dial will always simply be what you choose to include in your campaign. If anything, we might eventually see an even more simple and stream-lined version of the game marketed to younger kids. Any "advanced modules" will be possible options for the core game.
Going forward I expect that the game will continue to be centered on
stories and worlds, meaning adventure books and campaign settings. They'll sprinkle in a rules supplement every two or three years, such as more monster books and another Xanathar-like book, but the focus will be on stories and worlds.
I think the plan is to continue as-is until the
50th Anniversary in 2024. We'll see around 4 hardbacks a year, maybe expanded to 5, with 1-2 adventure books, 2-3 setting books and 0-1 splats, or something like 14-17 more books for the rest of 2020 through 2023.
In the next few years we will almost certainly see a
Manual of the Planes in some form or fashion, perhaps with options for
Planescape, Spelljammer and planewalking. We'll almost certainly see more
Magic settings (probably one every two years, with
Zendikar or
Innistrad up next, possibly
Dominaria et al at some point).
Psionics will either be in a
Xanathar 2 (with a different name) type book, and/or attached to
Dark Sun, which I think will be the next legacy setting, probably in 2021-22. Other legacy settings like
Greyhawk or
Dragonlance are possible but less likely and may have to wait until after (or with) the anniversary. We may or may not see a new
Forgotten Realms Campaign Setting book.
New classes that we might see are the
psion/mystic, warlord, maybe
shaman. I could also see more focus on
epic tier and beyond, although this might be later on. We'll also see another
compilation adventure book and more story arcs set in the Forgotten Realms. We might see a story arc set in another world, and a surprise or two along the way.
In 2024, we'll see
revised core books--not new editions, but something like "errata plus," or akin to a 5.1 or 5.2: errata, minor tweaks here and there (e.g. re-buffing a few monsters, a new ranger, etc), and new art, possibly including classic pieces from older editions (e.g.
Emirikol the Chaotic belongs back in the DMG). I think we could also see some kind of
deluxe anniversary box set that either re-formats the rulebooks in box-set form, covers the classic Greyhawk setting, or something else, like my pipe-dream product: a "Classic Worlds of D&D" box that includes short overviews of every D&D world.
After that, who knows--lots of unknown factors, both small (how D&D is doing) and large (how humanity is doing). I could see the game continuing as-is, possibly expanding to other genres (e.g.
science fiction or
superheroes). We won't see a truly new edition unless 5E tanks and/or is sold, at least not for the foreseeable future.
Or to put it in another framework, we could imagine it like the MCU phases, with each new one adding a new element but enfolding the previous ones:
Phase One (2014-17): Core books, establishing focus on story arcs.
Phase Two (2018-20): Expanding to include (new) worlds.
Phase Three (2021-23): Filling out the line with planes, psionics, and maybe epic.
Phase Four (2024-?): 50th Anniversary--revised core, etc.
Note that after the first year, Phase One saw three new hardcovers a year, while Two saw four; this
could mean that Three will see five. I could also see Two extending through 2021, and Three including the anniversary, with Four beginning in 2025.