If a plugin is useful and implemented on a popular blog you will get more visitors. If plugin become popular you may be amazed by the traffic. Just think what you are going to give is of someones use or not.
You can never judge how much traffic you can expect from any site. If your plugin has juice and something new, it will instantly become a hit like many other plugins have become but if your plugin is just another plugin it might tank down as there are many competitors out there for the same slideshow plugins.