There are various characters and shared universes that have been 'open sourced' in some way. Here are a couple of starting points:

https://pdsh.fandom.com/wiki/Category:Open_Source_Characters

http://free-universe.myartsonline.com/

https://paradisichronicles.wordpress.com/

https://www.thefifthworld.com/about/shared-universe

http://www.scpwiki.com/