| Are you still stuck? Well, here's some more tips:
Can't come up with a storyline yourself? Then don't! Just create a roleplay based on one of your favorite TV shows, movies, books, etc. Or perhaps you could take one of your favorite fandoms, change it a bit, and then make a roleplay about that "spin-off" fandom (e.g. a version of The Hunger Games where adults compete instead of kids, a version of Harry Potter where education at Hogwarts is "structured" differently, a version of Teen Titans where you can make up your own superheroes/villains, etc.). Just make sure to give that fandom credit - e.g. point out in the plot that you based the roleplay on something else, or simply name your roleplay after the fandom (e.g. "Harry Potter Roleplay," "Catching Fire," "The Avengers RPG," etc.).
Roleplays often consist of various adventures the characters go on called "Missions." However, other roleplays just have a single, ongoing plot that changes depending on what happens. Missions help provide more "structure" to a roleplay, making planning ahead much easier, but there are often roleplays where separate Missions just don't fit in.
You can have Missions (or simply segments of the plot if your RPG doesn't include Missions) focused primarily on one character, but this might be a bit unfair to other players. Therefore, you should also have other Missions (or plot segments) that focus on their characters to keep the rest of the players happy.
Try to avoid repetitiveness in the plot as much as possible. For instance, in one of my superhero roleplays, a villain forced one of the good guys to become a villain too. Eventually this good guy overcame the villain. In a later Mission, the villain forced the same hero to side with him in a different way, but it ended the same way. If you want to have something that happened before happen again in a roleplay, think about it for a while; try to use your imagination to change the thing that happened until it is fairly different from the original story but still satisfactory. Note that there are exceptions to this; sometimes having the same thing happen to the characters all over again can add drama and allow you to throw in more plot twists.
There are many genres to choose from; most of these genres are used to categorize movies and books, but they can be used to categorize roleplays. Some of them include fantasy, science fiction/sci-fi, horror, disaster/apocalypse/dystopia, drama, action, comedy, tragedy, thriller, western, adventure, historical, romance, philosophical (this genre is often combined with sci-fi), saga, crime/mystery, and survival (this is often a sub-genre of other genres such as adventure, action, and horror). When choosing (a) genre(s) for your roleplay, it's useful think about what sort of movies and books you especially like. You can identify what genre(s) those movies/books are in and pick (a) genre(s) based on that. Then, knowing what genre(s) your roleplay is in, you can then construct a plot based on those/that genre(s). For instance, I am a fan of comedy, sci-fi/dystopia, and action movies. I could create a superhero (action) roleplay, an odd but funny (comedy) roleplay, or invent an apocalyptic world (dystopia) and use it for a new roleplay. Or, I could perhaps combine these things; I could make a parody on the average dystopia (a dystopia-comedy hybrid), I could make a violent, jarring apocalyptic roleplay (dystopia+action), or perhaps a funny but battle-filled RPG (comedy and action combined). So, don't be afraid to try finding out what your favorite genres are and then mixing and matching them when coming up with roleplay plots.
Don't want to sift through the 900 million genres (OK, I'm exaggerating a little, but you probably get the point) there are out there? Then just think of things you like. If you like stuff like The Hunger Games and Divergent (young adult dystopian fiction), then create roleplays based on them or create a roleplay with your own dystopia that could perhaps be partially based on them. If you're an animal lover, then create roleplays where you play as animals. If you like survival movies/books, islands, mountains, and other sorts of wildernesses, then create a roleplay where people are stranded out in the middle of nowhere (on an island, in a forest, etc.) and have to survive until they figure a way out of there.
If you're really stuck when it comes to inventing a RPG plot, then do some research on the Internet. You might find some other useful tips and tricks. For instance, here is a site that contains the "skeletons" of common roleplay plots and plot twists that work everywhere: The Big List of RPG Plots Go ahead and read it. Then think about some of the plots and plot twists you liked, and using some of my previous tips (and your own ideas) go ahead and try to create a roleplay plot.