When will code completion for C# added to Godot?
C# IDE functionality is unlikely to be ever added to Godot. The current C# editor is designed for testing and performing small fixes, not writing an entire game using it. There are many high-quality C# IDEs out there, such as MonoDevelop, Visual Studio, Visual Studio Code and JetBrains Rider; it's not worth spending time implementing features which already exist in established editors.