Really loved playing the Great Circle, but now what?

As is the problem when you find something you really love, you’re left with kind of an empty void on what to do when you have finished the thing.

I’m currently playing through to try to complete the game 100% after concluding the main story.

Two questions: I saw mention of a secret ending but do you have to have 100% of the side quests done before completing the main to see the secret ending?

And now what do I play!?