I've always had lots of ideas bombing around in my head and writing helps to get rid of some of them. However when you're working on one story, that doesn't stop the different ideas dropping in when you're trying to figure out the best way plant a clue in a twisting plot.
Usually writing down the ideas holds them at bay, but sometimes it's worth taking a step back and taking an overview of what you're doing.
Recently my plan was to finish the first draft of my Florida Keys crime thriller before moving onto a series of short stories. I've been working on this story for about 6 months and have a pretty good outline fleshed and have written about 15,000 words.
But novels take time so I'd be looking at the end of the year before I finished the first draft and the short stories. So this time I've listened to the voices in my head and the novel has been put aside and the short story series is now my priority.
The main reason for this is time. Working full time limits how much I can achieve. That's realistic. I also want to feel the accomplishment and satisfaction of publishing a piece of work on Amazon. With the short stories that's realistic. Once I have a few books published then I'll feel happier about taking the time out needed to write a novel.
So now my focus is on my short stories. The idea is to do a series of cozy mysteries (think Agatha Christie novels!) and then publish a couple per book.
After reading some Dresden File books by Jim Butcher (who also offers excellent writing advice), and with an interest in urban fantasy, I think the heroine/amateur sleuth is going to be an anomaly - a non-magic human in a magical world.
The world building starts here!