This is what Dragon Ball Fanon Wiki was made for: your fan fictions. Consider this site your way of adding even more characters, plots, stories, and adventures into the Dragon Ball universe. You can base your fan fiction(s) off past events that were never covered in the series or create a page for a new hero or villain. You can even revise past adventures and give them alternate endings. You're only limited by your imagination.
Deviant Art is a great place to look for new characters created by talented artists to use for storylines, but then again it is more original if you create a character yourself from scratch. Be sure never to steal art from anyone, as that can get you banned. Always ask for permission first, commission an artist, or draw your characters yourself if you can! Your best bet is to search Dragon Ball Z and search under newest.
Dragon Ball Wiki is a repository of canon and official information about the Dragon Ball universe. You can use that wiki to help you get ideas for developing new plots or creating new characters, and it's a useful reference guide, both in terms of its timeline and character pages, which give detailed histories of every official Dragon Ball character.
Dragon Ball Fanon Wiki
There are thousands of pages on this wiki. The best way to see what other users have created is to visit either this page or this page. Remember, be bold and have fun while writing your fan fictions and reading what other users have created!