How we can define something like below:
func func_name() -> [A Custom Scene Class]:
... do sth ...
Scenes are not classes. Scripts are. I would guess it would be this if your script has a class_name:
func func_name() -> YourClass:
As shown here: https://docs.godotengine.org/en/3.1/getting_started/scripting/gdscript/static_typing.html#a-brief-look-at-static-typing
Yes! I add 'class_name' line on top of my scene's script and then I could use it as a type.