Unity editor menu item for creating interface. Place this file into 'Editor' folder.

CreateInterfaceMenuItem.cs

using UnityEditor;
using System.CodeDom;
using System.IO;
using System.CodeDom.Compiler;

public class CreateInterfaceMenuItem {
	[MenuItem("Assets/Create/File/Interface...", false, 10)]
	private static void CreateInterface() {
		var filePath = EditorUtility.SaveFilePanelInProject("Save file", "Interface", "cs", "Enter interface name");
		if (!string.IsNullOrEmpty(filePath)) {
			using (var streamWriter = new StreamWriter(File.Create(filePath))) {
				using (var textWriter = new IndentedTextWriter(streamWriter, "    ")) {
					var fileName = Path.GetFileNameWithoutExtension(filePath);
					var unit = BuildInterfaceGraph(fileName);
					var provider = CodeDomProvider.CreateProvider("CSharp");
					provider.GenerateCodeFromCompileUnit(unit, textWriter, new CodeGeneratorOptions());

				}
			}
		}
		AssetDatabase.Refresh();
	}

	private static CodeCompileUnit BuildInterfaceGraph(string interfaceName) {
		var codeCompileUnit = new CodeCompileUnit();

		var codeNamespace = new CodeNamespace();
		codeNamespace.Imports.AddRange(new [] {
			new CodeNamespaceImport("UnityEngine"),
			new CodeNamespaceImport("System.Collections")
		});
		codeCompileUnit.Namespaces.Add(codeNamespace);

		var declaration = new CodeTypeDeclaration(interfaceName);
		declaration.IsInterface = true;
		codeNamespace.Types.Add(declaration);

		return codeCompileUnit;
	}
}