Cool plugin! I like the unique feature it provides, so kudos to you for this.
I wonder if you would be open to a couple suggestions?
1: What do you think about adding a settings section, with the ability to add/remove messages to the loop of messages available?
2: What do you think about a feature, allowing message groups to be assigned, based on user Roles (I.E. Administrator, Editor, Author, Contributor, Subscriber, & No Role for this Site)?
My thought is to allow specific messages to appear, based on your Role on the site? Of course, this feature would need a way of being enabled/disabled.