VS Code utilises[1] OmniSharp-Roslyn[2] (which is an open source project not started by MS) to provide intellisense and refactoring for C#.
If you want to use something that is fully open-sourced (since day 1) and also utilises OmniSharp Roslyn, then check out OmniSharp Atom [3], which is an Atom plugin written mostly in TypeScript. OmniSharp Atom and OmniSharp Roslyn are both very welcoming of contributions. So if there's something that you see that's missing, feel free to send a PR :)
[1] https://code.visualstudio.com/Docs/languages#_c35