Just a random discussion point I have. There are story genres out there that have been milked dry and others with udders on the verge of bursting... that was an awful metaphor, but you get what I'm trying to convey.
Let's start with overdone genres:
For crying out loud, this has been done to death, yet it keeps clawing itself back up just like the convenient antagonists it portrays. Every variation of zombie, every character type, every scenario, pretty much nothing original left.
Whether it be apocalyptic or just an excuse for a mysterious antagonist (extraterrestrial, interdimensional, or otherwise), this is also a genre that's just dried out at this point. On the rare occasion, there's a unique scenario, like in Battle of Los Angeles where modern combat tech is pitted against extraterrestrial drone tech on a more-or-less even playing field, or even Avatar where humans are the alien invaders against a primitive species. Unfortunately, there doesn't seem to be enough creative juice flowing through this category of fiction.
Super people beating up other super people with no regard for collateral damage and giving everyone else an inferiority complex. Been there, done that. Moving on.
There are a few exceptions; things that have been going on forever yet always manage to get an original twist. One good example:
Finally getting a big-budget makeover with a unique setting while staying true to their original content. Appeals to nostalgia are worked in perfectly without dominating the plot.
One of the biggest things that long-running franchises have been falling victim to is equating originality to enjoyability. This is not always the case; you can have a completely original plot or arc for the franchise that undermines everything people enjoy about the franchise or tries to appeal to a small number of people that don't even invest in the franchise to begin with (take SJWs for example). Star Wars is one of the biggest victims of this, along with Doctor Who, Marvel Comics, and other unfortunate classics.
Now for the genres that haven't been explored enough:
Essentially a Victorian Era setting with a heavily industrialized and sometimes mildly futuristic twist, this is by far one of my favorite genres, in a tie with Dieselpunk for first place. Technology in this genre takes a lot of inspiration from Leonardo da Vinci's concept sketches, such as corkscrew propulsion, ornithopters, volley guns, and airship-scale hot air balloons. I'm amazed we don't have a proper multiplayer FPS in this genre yet (Guns of Icarus kind of counts, I guess, but it's less of an FPS and more of a ship crew simulator).
A similar concept to Steampunk, but focused on the Industrial and Modern Eras with similar hyper-industrialized or futuristic aspects. In many cases, this genre is based on how people in these eras envisioned the future, most notably in comic books from those decades. The movies Sky Captain and the World of Tomorrow and Captain America: The First Avenger are good examples of Dieselpunk settings. Iron Harvest is a good example of a Dieselpunk RTS game.
Deep Sea Adventure
To be fair, the Aquaman movie explored this genre a bit, but it was still primarily a superhero movie. Atlantis: The Lost Empire combined a little bit of this with a bit of Dieselpunk, but they weren't the dominant themes. Voyage to the Bottom of the Sea was a 1960s sci-fi series in this genre, but there hasn't been much else besides the Subnautica game franchise and a few horror movies.