I agree with what you mention the skills of the characters, in addition to the game being balanced with the guards, according to the character we choose. It's better than my idea that I couldn't mention to you before.
And I am interested in the mechanics of the ranks to be able to unlock new levels, excellent idea, I would like to try it.
It also occurred to me to add a story, but I never knew exactly what it would be about or how to add it. But if you added one, I guess we'd have to capture, confront or something like that “the boss or traitor” depending on how the story is. In the aspect of developing the story I do not think it is of much help, I am sorry, but, even so, I will try to give some ideas.
Answering your question, they are good ideas to keep playing. And what is the custom game option for?