OO! Utterly pointless and impossibly arbitrary subjective speculation, YES PLEASE!
Lessee...10 years down the road...what do I produce/my 5e bookshelf look like.
Well, ignoring all of the "broader brand" as already (wisely) incorporated, like video games and [gods please!] a decent cartoon (ya know that portrays an actual D&D world and party, tropes-a-go-go] and mechandising stuff (teeshirts, coffee mugs, dice cozies are utterly trivial to make these days. Where's my supplemental complete expendable income/impulse-buy random D&D "stuff"?)....ignoring all of that and looking at "printed materials" (whether or not there are partial or additional PDF versions of any of this, as I expect ALL printed stuff will be/have electronic equivalents in the coming 10 years)...
Year 1: A PHB, DMG & MM. A Starter/Basic Set. An adventure...turns out we have an "Adventure Path" (dragons/Tiamat). That's nice n' all. But more on that later. Got alla that already. Good job.
Year 2: Adventure Path #2 (elemental evil) is in the works/about to come out as we type. A "Fiend Folio" [a.k.a. MM2]. A campaign setting [hardback or box set, either or is fine with me]. I believe this is similarly already int the works and would expect it to be an FR supplement.
Again, most of this is already a foregone conclusion. So that's Year 2 done.
I, personally, would produce at least 2 "single [open to/cover 3 levels] modules" (as opposed to the 1-15 adventure path), one low-levels, one mid-levels, that have NO crossover/nothing to do with the adventure path material, for those who don't want to follow the "adventure path" or deal with elemental evil.
Year #3: So. Now...I would do (though probably wouldn't use) a third adventure path, levels 1-15, undead and/leading to liches, vampires and death knights [oh my!] focused. Tie in a trip to Ravenloft and, eventually, Acererak in there someplace. There's your Year 3 path project.
3 "single modules": 1 low level, 1 mid levels, 1 high (12 or 15+) levels. No tie in. No crossover. No undead Big Bads.
"Deities & Demigods", with, maybe Norse, Celt, Egyptian, Babylonian as guides for using "real world" mythologies, guidelines for animist religions, poly-, mono- and pan- theistic religions/societies, and then the FR, Greyhawk, Dragonlance, and...I dunno...Eberron mythologies for "World of D&D" mythologies. [And against any of my personal taste, but popular enough to recognize and exploit for sales] Section on Sigil & Planescape setting. That's your "big new release", as now adventures/adventure paths are to be expected, for Year 3.
"Psionics Handbook". Not nearly the encyclopedic tome DD is, maybe half the size and price. Psionics in 5e. Including some tidbits, maybe an appendix-to-not more-than-half-the-book about the "World of D&D" Dark Sun campaign setting.
With 3 years of play, and APs going up to 15, higher levels are to be expected, so a "World Builder's Handbook", en par for page count with the Psionics book. Cover castle/keep and nation building. Governments. Large scale combat/war rules/guidelines. Same amount of pages devoted to a "World of D&D" Birthright and/or Dragonlance (provides a little synergy with the DL's DeitiesDemigods material) campaign settings.
Year #4: "Year of the Attack of the Settings", following up on the momentum of Deities & Demigods. Greyhawk setting set/hardback. Eberron (though, again, I personally don't care for it, but I'll publish it) setting/hardback.
Year we skip the adventure path. d4+2 single modules: 1 low/2-4 mid/1-2 high levels.
Year 5: Amid the din of congratulatory praise and desirous cries for "MOAR!" by the raging public, we celebrate a "5 Years of 5e!" anniversary in regal form, with a colossal rewrite/expansion of GDQ in hardcover adventure path "Against the Descent of the Giant Drow Queen's Demonweb Vault of the Depth Pits."
5e's "Unearthed Arcana" introduces 5-10 new optional subclasses/archetypes, 5 optional new PC races (whatever hasn't been covered yet in their respective setting-specific books), spells, magic items, feats, combat-additive stuff, etc... All optional stuff.
Year 6: When the complaints about why don't I have a planescape book to go with what Deities/Demigods said about it have reached fevered un-ignorable annoyance pitch: Planescape setting book/set.
d4 Single modules that have nothing to giants or drow, at least 1 should be for Planescape.
"Wilderness Survival Handbook": For those not interested in interplanar shenanigans...all kinds of optional adventuring environmental stuff/rules. Maybe a new subclass or two. A couple of sylvan/monstrous races. Like the Psionics & World Builder's books, some-to-[no more than]half page count devoted to exposing/exploring options to use "Worlds of D&D"...I dunno...who goes best with "wilderness" stuff who doesn't already have a setting book? Mystara? "Nentir Vale"? Some new/independent? Oo! Maybe Blackmoor?! But some "Worlds of D&D" campaign setting that goes well with using the wilderness survival stuff.
Year 7: d4+1 single adventure modules.
Year 8: When everyone's complaining about how nothing came out last year, "Book of Spells Handbook" & "Book of Swords Handbook", en par page count and content-wise with Psionics & Wilderness handbooks. One focused on magic-using classes, one on non-magic-using classes [if we haven't seen a spell-less ranger before now -and I would be amazed if we didn't- it would go in here].
Year 9: ...meh....guess it's time for another adventure path: "The Sinister Slavers of Dunwater Stockades of the Enemy Lord's Aerie's Undercity of Saltmarsh."
Year 10: "10 Years of 5e!" Super celebratory 10 -year special limited anniversary edition/printing of 3 core books, the Tiamat and Elemental Evil books....while sitting back to count my millions garnered over the past decade.
SO, after 10 years, my 5e bookshelf looks like this:
Top Shelf: PHB, DMG, MM, FF, DD [potentially Anniversary editions of the PHB, DMG, MM]
Middle shelf: A Psionics (w/Dark Sun) Handbook. A World Builders (w/Birthright/Dragonlance) Handbook. A Wilderness (w/Mystara/Blackmoor/whichever is best suited) Handbook. A Magic/Spells Handbook. A Weapons/War Handbook. FR setting guide. Greyhawk setting guide. Eberron setting guide. Planescape Setting Guide.
Bottom shelf: Dragon/Tiamat AP. Elemental Evil AP. Ravenloft/Liches AP. Giant/Drow AP. Saltmarsh/Slavers AP. And roughly 10-15 individual "a few levels module" adventures. [potentially "Anniversary" editions of the Dragons and Elemental APs, though I probably wouldn't bother.]
Do note: this is not all stuff I would WANT or, necessarily, enjoy, but it is all stuff I think would make enough people happy to warrant printing stuff up for them to get and...well...take their money.