I know that theoretically one can put what he wants in the method, but I wanted to know if the method should not implement any logic other than defining a string with the next page that should be loaded.
For example, is it conceptually right to put values in the flash scope there?
Methods bound to actions act as ‘controller’ part of MVC triad. So I’d say yes, it’s a proper place to put things concerning the workflow through your application (like setting values in flash scope).
You should probably refrain from placing business logic there – those things are best left to separate business classes. That way you achieve nice separation of concerns, making the whole thing much easier to understand and change in the future.