Well, I was thinking something like a single-player game where you are the spider and, through the course of whatever the gameplay entails, you struggle as a spider to stay alive. This could perhaps build empathy for spiders and cause people to think twice before casually killing one. So beyond conquering arachnophobia, what if we can give people a positive affinity towards spiders?
A multi-player spider game sounds potentially terrifying.
Likewise, if you have a fear of spiders bad enough that it interferes with your quality of life, you likely want to be desensitized to spiders.