@Ross at Play
I am also writing several stories that I am finding difficult get to a clean end
I think that's a different problem to the one others have been trying to solve here.
Opening a universe is more than just finding a suitable ending to a single story. Instead, you're forgoing every completing the story, even if a brilliant idea occurs to you later on. They key to a universe, is that NO ONE can change the current status of the universe (i.e. no solutions or resolutions which might preempt all the other stories).
I think Ross's idea is your best bet. Ask the other authors for their opinions on story options. If all else fails, you can always ask your readers, though they tend to inundate you with competing solutions which don't always fit the story you envision.