Check the IDE screenshot running on the console: https://dlang.org/blog/2016/10/07/project-highlight-dlangui/ https://dlang.org/blog/2016/10/07/project-highlig

Check the IDE screenshot running on the console: https://dlang.org/blog/2016/10/07/project-highlight-dlangui/ https://dlang.org/blog/2016/10/07/project-highlight-dlangui/ It is written in D, a fast (close to C++) language, easy to write code in, runs in a terminal, runs graphically, runs on many systems (I don't know every possible system but I doubt in runs in a Commodore 64), it is open source and I have no idea about it being extensible or not. It's however the closest thing I have seen to your list of requirements.