It’s All One Book

This could be a multiverse, it could be a long introduction to timelines that start at all sorts of places and converge into one eventually, it could be the many lives or incarnations of the main character.

Shove them into one place.

A shitty portfolio. A long list of good ideas. Let them brew in the same place and bleed into each other.

Then have your friends and family guess the endings. Have readers predict how you're planning to end things, and then agree with them. Tell them they're clever and they figure it all out so perfectly. Agree with them and accept that their predictions and suggestions are canon.

You're going to hate their answers. None of them are going to sit right with you. They're missing too many details you hadn't developed into the story yet, they're skipping over some of the arcs you hadn't completed yet, and all in all, they're endings that don't do the characters justice and waste the potential.

Let that dissatisfaction motivate you to write in the proper endings, and bridge the end to where you left off last. Fill in the blanks, but predestine the endings. The middle is where you have the most freedom, so set that endpoint and deal with the middle for as long as you can