I used to struggle with coming up with a concept I thought was worth writing about. I knew that I was good at using words descriptively, etc. but I couldn't take any idea past the 10,000 word mark because I didn't feel that I knew how to plot, or I thought there was no great meaning behind what I was writing (analyzing literature in school really screwed me up for years).
I thought back on when I really enjoyed writing. My fifth grade teacher would give us a title and we'd all write a story based on that title. I wrote the most ridiculous, Indiana Jones-like adventures and I loved it! So I gave my twenty-something self permission not to try to write the next "great novel," or even to worry about the plot and just let myself go. The result was a middle grade science-fantasy written in the fun-loving spirit of my little-kid self.
Once I finished that novel, the stories just came pouring out, and they show no sign of stopping. And, no, my other stories aren't silly. The latest completed novel is for adults, and has an intense, serious feel.
In my experience, writing a novel without worrying about plot, theme, etc. (It ended up having both an engaging plot and a strong theme anyway) enabled me to move forward and write other novels in which I did worry about those things ahead of time.
My advice: write a practice novel, just for fun. Don't worry if it's really bad. Practice not being perfect, and your writing may improve!