how about a combination of things you've drawn before?
like if you've drawn animals and you've drawn people, draw them together.
i draw mushrooms with horns and weed stuff, so I like to draw mushrooms shmoking weed haha
if you've drawn girls, draw them in new settings, like a bar if you've never done that.
draw a beautiful girl in a red dress with her back to the bartender, facing the viewer and crossing her legs. and draw a bunch of guys in black, gray and dark colors around her, just marveled by beauty. and a naughty little smile on her face.
Are you talking about drawing flash, or submitting portfolio to get a job or apprenticeship? Usually traditional stuff is best when drawing flash. Look up some Sailor Jerry stuff. Also, custom work is always appealing to people who don't want the same tattoo 100 other people have. Draw something in your paricular style to show what you can do and offer custom work--people will pay more for it.