If you want to see all the content in the game, follow up any quests that are marked with a blue 'plus' symbol on their quest markers, because they'll always start you on a quest chain to unlock something or other. There are a lot of optional dungeons and raids that you might have skipped along the way, and some of them tie back into the main story. Have a look around the various capital cities plus Mor Dhona.

For actual endgame stuff now (and assuming you're up to date with MSQ), you'll want to unlock:
- the two optional dungeons, the Twinning and Akademia, which will allow you to access the "Expert roulette"
- the Eden raid series
- extreme trials

Most of the unlock quests start at the Crystarium, but the Akademia quest is in the Eulmore slums.