Insert a blank space into the timeline

Depending on how much "stuff" (love those technical terms) you already have on the slide and therefore the timeline, you could try shrinking the view percentage (hold the control key and scroll back with your mouse wheel; at least that's what happens for me) to allow you to see everything in the timeline with space at the end. Then select all of it, and you will likely find that it can drag to the right... giving you that 15-sec window of fame, waiting to be filled with all sorts of creative goodies...

Zoom back in to your normal view, and do your work...

When your masterpiece is built, scroll back out and then grab everything you moved "right," and move it to the left.

You might try this with a slide that doesn't exactly have a lot of "stuff" on it to see if that gives you the effect and results you're looking for.

Good luck...