From a WIU perspective, we consider Illinois State a "mild" rivalry...mostly just because of history. ILSU has an actual rivalry series/trophy with EIU though.
Used to be WIU and EIU were fairly big rivals, but hadn't really been much of anything after EIU left the Gateway conference back in the mid-90s. I'm hoping that picks back up now that we're back in the same conference together for all sports.