If you have a storyline, I suggest you write it down somewhere in case you wish to write it in the future. However, I suggest not to write too many new stories as the old stories which are yet to be completed would be neglected. Keep the ideas until you're certain you have a whole story which satisfies you then post it up whenever you feel like. :)

You can write another fic but in order to finish the other fics, try to write the outline of the new fic. Get a main idea of it and understand the plot line. Take note of it and save if somewhere so you won't forget about it.

Then, you continue your old fic.