Hi
First of all that is not a lame tattoo – that is quite a nice sugar skull you have there and all it needs is a small tidy on the outline.
What style did you have in mind for adding to it?
If you were going to keep the same style I would suggest possibly death mask butterflies (where their wings form a skull).
You could have everything from flowers to poison chalice or a qoute underneath – really need to know your likes and dislikes to help further.
Take care
Matthew