Their are more worlds known that are open to tourists, some are too dangerous for tourists, be they ravaged by war or disease. On some civilization has fallen apart. Some are even crazier than toon world. Others are too close to our world to be interesting. Some are like the out of control teen gangs of the pulp novels from the fithties to eighties. In some what is left of humanity belong to cults worshiping real gods. In some myth, fantasy or horror things are trying to fit in with the modern world. In some aliens have concurred earth or Earth is just one world in a league of worlds. In some the wild frontier has moved to the asteroids. In others things are reversed, like anthros own female human pets, boys grow up to be women, toys play with kids. Some worlds have been over whelmed by supers. Others have technology it would be too tempting for some tourist to steel and bring back. In others the Earth have been over run by zombies, vampires, werewolves. Some are a mishmash of cultures, like Native American and Vikings. Weird science could rule. Lovecraft like steam punk. Some look so much like past eras that contact would change that world forever.
One set of time lines seam to steam from time travel was invented in the Forties or people were sent back in time to prevent or erase a horrible or dark time line.
Open world action group is a group that thinks more than one company should control multi world travel and that worlds we could learn from are being hold back from the public. John and Carrie are dropped in what forbidden world?