2: It is Your Game
It might be their system, but it is your game. At some point you have to say “we start here”. From here on out everything has to audition to get in. Just because Turkey Shit Rules or Lizards of the Coast wrote it is not a free pass. YOUR GAME. Even if you use a published setting. YOUR GAME. The next supplement for the World of Weeniecraft does not get a free pass, you need to approve it no matter how kewl Player X thinks the Hot Dog Cart of Doom is. YOUR GAME. If you want a good game you must take ownership.
A Linnaeus wrote an article titled Crucifying Elminster as a way of establishing a prepackaged setting, to wit Forgotten Realms as your own. Literally, the PCs find Elminster crucified outside the city walls. I consider that an extreme example. I admit that some players might require the demonstration. It’s a little rough on poor ol’ Elminster.
This is obviously only an issue with the aforementioned packaged setting. The easy and hard route is to roll your own. Create your own world from the get go. After all Ed Greenwood did not wake up one morning and have all the books and box sets sitting on his table. Likewise you do not need a world fully fleshed out to start a game. Detail a valley with some interesting points. Rough out the area around it. Build up the land as required, the gods, the kings and so forth, well if Fantasy. You can do the same thing in a post modern New York, you might still have a king even.
I make a point to keep some areas loose enough that I can drop a pre-written module in if I want to. Roll your own does not mean rolling all of it. I’ve made extensive use of Dungeon magazine (when it was a thing) and other packaged products. Seahaven is the Waterdeep map rotated 180 and dropped into a very different landscape. Even with that the map has undergone some real changes. I would not trust an old Waterdeep map to get you around.
And, if you are not publishing, there is no plagiarism at the game table. Ideas are were you find them. Grab gleefully with both hands. I keep my serial number file handy for such incidences. The Old School tools, you use what you find, genre and author aside. Oh yes, cross genre all day. A good friend loves Star Wars because he can turn anything into a Stae Wars game. The setting is very flexible.
Conclusion? Play it your way. Packaged setting, roll your own or home rolled published world. Take a setting and make it your own.