Too much of the time, fiction revolves around a dozen time-tested well-worn plot devices. If you're like me, you want what you write to be unique, different, and very compelling. Observation is going to be your best friend when coming up with new ideas for plots.
For example, just observing my wife planting bulbs gave me the idea for a new fantasy story. She was planting bulbs, and I was fascinated with the way that a bulb grows. I thought, "What if a human being grew the same way? A person who apparently grows up and grows old and dies, but then is reborn the next season. A person who appears to be 'normal', but in actuality is...a flower." With that new idea for a plot, I was able to write a short story about a doctor who created a creature that followed the life cycle of a bulb. It is unpublished up to this point, but it was fun to write and research.
Another device I use to help jog my brain into new ways of thinking is to brainstorm titles. I come up with as many interesting or bizarre sounding story titles that I can, and then I set out to create a story based on the title. "Spider Faucet" was an interesting story that grew out of just a title. So was "Always Comes Through In A Pinch".
The second best friend you will have in coming up with new ideas for plots is the non-fiction area of your local library. Learning about something heretofore unknown has been a great way for me to explore new pot devices and ideas. I tend to enjoy ancient history, and reading about the average Roman citizen's devotional life led me to combine a bit of Dickens and a bit of pagan Roman theology. What I ended up with was "A Saturnalia Tale" ... a retelling of Dicken's "A Christmas Carol" only transposed to ancient Rome.
Much of what I write I do for personal enjoyment, rather than for mass consumption, but occasionally the two worlds intersect. It is very gratifying to be able to make a few dollars on something one enjoys to do. My personal goal is to always write a plot twist that hasn't been written before. The other thing I try not to do is to read a lot of other people's fiction, mainly because I do not want my voice 'infected' with another writer's voice. If you read a lot of Stephen King, you start to write like Stephen King...at least, I do. Which is why I shy away from reading any kind of fiction while my writing sap is rising.
A final idea for you is to go back through your old 'slush pile'. THis is that collection of unfinished or rejected material. If you read through that pile with fresh eyes, you will undoubtedly come away with some new ideas for finishing or improving your work, or for something brand new and unwritten. All the best as you write something that you would enjoy reading.