I would say start clearing quest. There are a lot of dungeons that are not part of the MSQ, so if you want to get the most out of the pre-expansion content, start checking out the quests in the post-lvl 50 areas (mostly Mor Dhona) and start unlocking the remainder of the dungeons and Trials (Odin and the Extreme Primals, for example) that you may have missed in the MSQ. For one, you will need them come expansion if you want to run Hard/Ex roulettes on a daily basis, and two it gives you more variety and will help prepare you for the upcoming content. The post-50 dungeons have a different speed and feel from the leveling dungeons. It will be good to get into the groove of them.