It would be great if patrons could (if the library wanted) create groups of libraries they like to search together. This would be like the groups that you can create in admin, but specific to each patron so they see it when they log in.
Instead of replicating the library groups feature, it might be a little more interesting to allow users to define custom search filters now. This would be more flexible and not only allow for a library limit, but also for other limits. But not sure what the best way would be to make this super user friendly? You cannot easily create a search for multiple libraries from advanced search. Only single and pre-defined item groups.