That's what I call corporatist-shaped imagination. What a pity the minds of the newer generations of gamers were shaped in such as a way as to express disdain and incredulity at the suggestion of hardwiring a homebrew culture into the game.
I express disdain and incredulity at the suggestion that DMs should spend time create half-assed worlds when what's there is better and saves the DM time to worry about other parts of the game. I could express the same pity that you actually play D&D instead of making up rules for what you need on the fly.
I understand you use "generic" to mean "medieval fantasy".
No, I don't. I don't know what medieval fantasy is, but I'm hard pressed to recall a truly medieval fantasy world for D&D. Generic as in Forgotten Realms, Greyhawk, Golarion, Harn, and Dragonlance.
1) Making use of whatever RPG books we already happen to own, regardless of which edition, and which company, and which world they're set. For example, besides the 5e Basic Rules, my entire game library consists of the Pathfinder Beginner Box, a True20 book, two Blue Rose books, a One Ring book, Dragon Age, and DCC RPG. That's plenty.
So in other words, you aren't the market for the products you demand. Frankly, that's an awful collection; virtually nothing there strongly backs up the 5e Basic Rules. You mentioned pushing someone to buy 60+ books for Forgotten Realms, but instead of what you bought, you could have bought a main campaign setting and a close look at one area and its surroundings.
I started a new 5E campaign last week which begins in Sandpoint (from Golarion)...but the rest of the map is not going to be Golarion.
Why? I could knock out a small town like Sandpoint in 15 minutes. Maybe I'd steal the map, but just copying it wholescale separate from the rest of the universe seems incredibly derivative without the advantages of that derivativeness.
2) If one sees a product that one really likes, but which isn't set in the same world, then it'd make sense to just stitch its map to your world map. For example, sticking Freeport in the ocean west of the Sword Coast.
And instead of ending up with a coherent world, you end up with a mess; what are gnomes like? why do we have completely redundant gods? why did we travel west to fight the orc invasion when west is Glantri?
I suggest four options to be included in the DMG:
1) Invent an entire world from scratch, with in-depth pantheons, world map, and one or more unique themes, so that the geography of the world and the trajectory of the campaign are more-or-less laid out from the start.
The World Builder's Guide comes to mind; it's a shame it wasn't released in PDF. It's an ambitious goal, that really needs to be a goal in and of itself; it's unlikely to be worth in-game the effort it took to make it out of game.
2) The same, but randomly generated by rolling on worldbuilding tables. (Like how scif-fi rpgs often have a planetary generator.) Roll for campaign setting themes too. ("Roll d% three times: Okay, our world is a gothic horror theme, with a Roman Empire-style civilization, where gnomes are the primary race). Roll for world names ("Jarth", "Grynn", "Aveir"). Roll for continent names ("Maerun", "Yoerik", "Ferilia"). Roll for campaign setting names. ("Bluehawk", "The Forgotten Sun", "Azure Realms")
No. Just no. It has all the disadvantages of playing with a DM whose wild ideas have exceeded his ability to make them concrete and playable, without the advantages of the DM actually having some vision. Taking a DM who doesn't know what he wants to play and handing him "a gothic horror theme, with a Roman Empire-style civilization, where gnomes are the primary race" is a sick joke that nobody who has to play with him is going to take seriously.
4) Piece together a world which is gradually stitched together from whatever adventures one happens to own, and which are only stitched together in the course of actual play (instead of being layed out ahead of time.) This is method I'm most interested in.
Okay. I don't think it profits players or WotC to encourage this too much.
They decided to use the Celtic pantheon for the gods of this world. Then the 5E War of the Lance adventure path came out, which included an appendix for converting all of the adventure to each of the published worlds
So you're taking a story centered around a certain set of gods, around a certain history, and think you can wave your hand and substitute a whole new set of gods and new history, it won't seem weird at all? How do you adapt an adventure path that depends on nobody getting spells from the gods for 300 years to Faerun anyway?
B) Invent an entire world whole-cloth, with its own pantheon, all new names--even to the extent that one is writing as if one were a copyright lawyer, making sure that no name or concept could be the basis for being accused of plagarism (which is how professionalized authors have to be in our corporatist culture). Don't use any published adventures either. Hand-write all adventures from scratch.
That's a heck of a strawman. You're welcome to invent whatever world you want, and drop whatever adventures in you want. But worldbuilding is more then just tossing a bunch of disparate parts together.
I don't think that hacking together preexisting pieces makes a more fun world for players and DM then playing in a world that is in some sense a coherent whole. I'm pretty sure history backs me up in saying that the time and effort spent building a generic D&D world does not pay back at the table, though it may be fun as a thing in and of itself.