This is one rumor I hope is true: on the Connected podcast, MacStories editor Federico Viticci said that, based on “conversations with others,” mouse support could be part of the AssistiveTouch accessibility feature.
"What I heard is without adapters, you will be able to use any USB mouse on your iPad, but as an accessibility device," said Viticci. "The iPad Pro has a USB-C port, so just plug in a USB mouse and if you have physical impairments, if you have any other kinds of motor impairments, just use a USB mouse in accessibility mode."
iOS 13 will almost certainly be previewed at Apple’s Worldwide Developer Conference in June. The end user version will probably arrive in September, along with new iPad Pros.
Chances the rumor is true, according to the Sellers Research Group (that’s me): 95%. I think it’s got to happen if Apple is going to push the iPad Pro as a laptop alternative/replacement. However, support for wireless mice makes more sense than only offering mouse support via a USB-C connector.