Err, actually, that is exactly what Ryumo or whatever the Japanese dragon is called has tried to do. He tried to syphon off mana energy from the Ring of Fire, to accumlate enough to transform into a post-Great Dragon state. he failed, and that is why Japan was so especially hard hit in 2062, but in any case if it necessites accumulation of so much mana, then it cannot be natural. Hence we can assume there is no natural state beyond Great Dragon state.