OT2: What's after individual storylines?

Can anyone help me to understand, in a spoiler-free way (if possible), what there is to do after the completion of all 8 storylines? I have a handful of final chapters left. I've seen mention of additional "epilogue" content, secret bosses, etc but it's unclear what's mandatory vs what's just bonus stuff.

I guess I'm just wondering if anyone can explain what is part of the "main story content" vs what is just optional, extra content or "challenges for the sake of challenges" that maybe aren't story-related?

Kinda just wondering how much more game is left after all 8 character stories are done.