MONSTR is a multiband stereo imaging plugin, available in VST3 and Audio Unit formats. MONSTR allows the user to control the stereo width of a sound in up to 6 different frequency bands, and so can be used to perform common tasks such as narrowing the bass frequencies while adding width to the highs.
For more details and a free download of the compiled plugin: https://www.whiteelephantaudio.com/plugins/monstr
JUCE: https://www.juce.com/
WECore: https://github.com/jd-13/WE-Core
VST SDK: https://www.steinberg.net/en/company/developers.html