The book shelf in my home office is a modest one. A simple put it together yourself model from the local outlet of the worlds largest retailer. Five shelves, plain white, and rather boring. I did replace the cheap cardboard back with salvaged pallet wood to add character and strength, but the shelf is just the vessel, what is on the shelves is what matters.
The top f the unit is covered in mementos, including a green telephone from my grandmother's house, a Spider-man cake decoration from my 30th birthday, and a ceramic dragon painted by my son when he was younger.
The first shelf is filled with works of fiction, many authors, many genres.
The second and third shelves is home to my gaming books, mostly GURPS but a few others thrown in for good measure. The books are sorted by genre. Fantasy, Alternate earth, Space, Western, Supers and a few Genre defying. In each section there are two or three large three ring binders.
Each binder is home to a campaign I have run in the past. Character sheets, NPCs, maps, all of it. Ready to go in a moments notice. The binders are the equivalent of a Zombie Apocalypse "Go Bag".
If my group of players were to come to me and say they wanted to change games, I could, in just an hour or two, get them running. By the time the players finished building characters, I can review enough of the material to run a starting adventure.
I have a fully developed Supers world ready to go. It pulls a lot of stuff from Champions and Mutants and Masterminds, but I run it in GURPS. I have 3 distinct power levels set up for the game, depending on what the players what to do. I can run a street level game, a regional/national level game, and an international/world level game.
I have a Fantasy world ready to go as well. I have several plot lines, each in a different geographical location, ready to go. The word has large cities, wild regions, and lightly populated areas.
I have SCI-FI covered as well.
Cyberpunk, got it, in the form of a mash up between GURPS Cyberworld and Shadowrun, depending on if the players what magic or not.
I can run a GURPS Reign of Steel, or GURPS Monster Hunter
I can run a Space based game too. Traveller, Firefly, WH40k, all ready to go. OK the 40k needs some work, but not much. With a bit more time I could pull together a Star Wars game, but that is very undeveloped at this time.
Point is, save the stuff you have already run. You may need to shave the serial numbers of a few things if your players have seen it before, but you will have it.
If your players aren't as excited about a game as you are, you can change with little fuss. If you need a one shot because the regular GM is down with the flu, pull out an old supers encounter to fill in. Need a break from the current game? Pull out the binder from a different genre. Stuck for an idea? Go through the binders, maybe change a weird alien into a new fantasy race or monster. Maybe the Pirates from you old fantasy game can crew the Star ship attacking the PCs.
Truth is other than the mega dungeon I am running now, I haven't run a completely original game in years, maybe decades. But, then again, I could drop it into Cyberpunk as a sewer, or Space as a space station, or Supers as an underground lair for the evil mastermind. Good thing I am saving all my notes in a 3 ring binder.