Finish off the rest of Broken Valley; you should be at least level 13 or 14 to get through Maxos temple.
After Maxos temple you will get access to Sentinel Island, where there is some stuff to do before you can take over the battle tower and get the dragon stone. Once you do that, though, most of the quests and areas in Broken Valley and Sentinel Island will be unavailable (there is one quest in each region that can be done later, which should be obvious by the level of the opponents, or the need to be able to fly).
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