If you’re one of those people who can come up with lots of cool backstory for your MC, but then runs out of ideas for the actual plot of the book, please allow me to hassle you a little bit.
You already have a plot!
As an example, if your MC was abducted as a kid and raised by pirates, and you want her to have her own crew at the start of the book…
Tell the story of how she learned to be a pirate and eventually mutinied against the captain who raised her! Put your desired starting point at the end of the book, then write the second one with the first as a foundation.
She gets her crew at the end of Book 1. In Book 2, maybe the captain she marooned managed to get off the island and is building a fleet to wreck her, or she’s being challenged by all the enemies the previous captain managed to hold at bay, or the fucking kraken sinks her ship and she has to swim to shore and start over.
If you have backstory, you have a plot, I promise. Just start the story earlier in the MC’s lifetime.