I tend to prefer breaking from canon to mainly come in the form of additional characters or changes to backgrounds rather than events which change the history of the whole region because that can dictate what anyone else willing to play a Stormlander house might be able to do.

That said I'm not dead set against it as the Stormlands are a pretty fractious 'kingdom' during the earlier Targaryen period. What might be better is rather than rival houses uprising against House Baratheon would be a rebellion of peasants combined with a particularly succesful Cornish raid? Maybe a new Vulture King of the Red Mountains. Just a thought.