Now, this theory doesn't exactly have much basis to it, but its an interesting thought nonetheless, and I thought i'd share my thoughts. :cigar:
Butterflies can symbolically represent the blossoming of a person going through the larvae > pupa > butterfly stages. Blossoming is of course also linked to flowers, the name of Robin's fruit and also one of her biggest passions. Oda has drawn the butterfly tattoo so that we only see half of it. This may be Oda's way of saying that Robin is still only half blossomed, and that she still has plenty of developing left to undergo.
But what if it also hints to a future fighting style of Robin's?
What's interesting about Robin, is that her fighting style is actually very physical (something easy to forget), only its often not her real body doing the fighting. In the past, she's had the habit of standing stationary while fighting opponents, letting her other limbs do the work. This I believe is a major disadvantage to Robin's fighting style. It's fine when fighting fodder, but in the New World enemies will be far deadlier, and Robin will need to bloom.
This is all pretty obvious stuff so far, but here was something interesting I found.