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Replying to my own prompt; personally I've been inspired by the lack of proper gyms.
My hack still remains firmly entrenched in the League story frame, but I've been trying to figure out a way to throw out the Villain Team hang-up driving the plot. As a result, the League Challenge aspect becomes more important to the plot and potential for 'side-quests' to acquire forward-progressing key items opens up.
Pic related is my draft of Cinnabar Island, where Blaine holds court deep within the dormant volcano.