This is what Dragon Ball Fanon was made for, your Fan-Fictions. Consider this site your way of adding even more characters, plots, stories and adventures in to the DBZ Universe. You can base it of past events that were never covered in the series or create a page for a new hero or villian. You can even revise past adventures and give them alternate endings. Your only limited by your imagination.
Resources-
Deviant Art-
Deviant Art is a great place to look for new characters created by talented artists to use for storylines, but then again it more original if you create a charater yourself from scratch. Your best bet is to search Dragon Ball Z and search under newest.
Dragon Ball Wiki-
This is where all the real and original stuff in Dragon Ball history goes, use this as a guide to give characters new adventures or new developing plots, even have old villians come back from the dead to get revenge. THe Dragon Ball wiki is for users who want to get information on non-fan fiction.