I did some proof-of-concept work on having Mathematica Front-End (MathSessionPane) in the IntelliJ Idea as a plugin https://github.com/dubrousky/Mathematica-REPL. It uses JLink as a runtime dependency and is available as plugin. Does it make sense to include it somehow into the existing Mathematica Support plugin (https://github.com/halirutan/Mathematica-IntelliJ-Plugin) or keep it separate? Can it be considered as a duplication of Workbench functionality that is prohibited by WRI licence?
