One thing I would discourage against is adverting your roleplay as a genre piece. Whenever I see "VAMPIRE RP" or "CRIME DRAMA RP" or "WESTERN RP", it makes me wince a bit. A good story worth roleplaying, in my opinion, is one where the character is compass; it doesn't matter if the setting is swords, shields, spells, or laser rifles - if the character interactions, their struggles can't be adapted to another setting, then you don't have your story built the right way.
Commit to your premise before you commit to your plot. Write a log-line for your roleplay! That is, define the characters, define their goal, define their conflict, and define the consequences for meeting (or failing to meet!) that goal.
Scumbag is right, too, though. Following along with a heroes' journey-style story model will give your story direction, and prevent that awful Gobi Desert of a second act from dragging you down. Believe it or not, that's often what grinds most roleplays to a halt, even after all the trouble of finding people to join - it's all the characters stepping together, and realizing, "now what?" .. that horrible moment when there's no flow, no ebb to the story.
Start with who your story is about. Then think about where you want it to go. Remember, you're the GM - it's only right that you have a better idea of this than the players. That isn't to say that you shouldn't be on the journey with your players, but you ought to have the map and know where everyone is going, rather than just going on blindly yourself - as is often the case with genre RPs.
Hope this helps!
-VV
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