The source of SS+ is available to those who want to use it. The license says all rights reserved but I'm waiving those rights as of now.
The IBB system, the ships and weapons, the Cabal, the Arcade, the new missions, and the variant randomizer + market injector will be retained in various mods. Graphics-related features are split off into GraphicsLib, i.e. renamed ShaderLib.
This means that the campaign changes, (vengeance fleets, economy tweaks, event changes, etc.) and skill + combat balance changes are being thrown by the wayside.