This answer wiki can not help you create an original character, however it can give advice.
This is a lengthy guide but it provides some tips: Fanmade Vocaloid Creation Guide. Another is the (title)loid concept, for that see Types of Vocaloids.
NOTICE : Do Not Create anymore fanmadge pages on the Vocaloid Wiki, because it is focusing on official and established information, please create a page at Fanloid Wiki instead.
Many people in the fandom choose to spawn a fanmade clearly inspired by Crypton Future Media (in particular Hatsune Miku) the arm sleeves, school uniform-like top and bottom, and boots or leg warmers. Do note that Crypton may be popular but they are not the standard of what a Vocaloid should look like, so you should try to expand by coming up with a fashionable design on your own.