There are a few styles I'd love. My favorite are probably the suits like this one:
http://www.ebay.com/itm/Girls-Princess-Rapunzel-Monokini-Swimsuit-One-Piece-Swimwear-Swim-Costume-SZ-3T-/190919805682?pt=US_Baby_Toddler_Girls_Clothing&hash=item2c73b50af2
It would be difficult to get done, of course, but there are sites that print images on swimwear (usually for swim teams, though)
The second style I enjoy:
http://www.ebay.com/itm/6T-New-baby-toddler-girls-Swimwear-Leopard-bikini-kids-Swimsuit-hat-HOT-SALE-/141085511220?pt=US_Baby_Toddler_Girls_Clothing&hash=item20d95a0e34
Skirted, ruffled, any combination of the two. Especially if the skirt is high enough to still show off the bottom, so you can have something peeking out under the suit.
Finally, there's those swimsuits with the little ruffles above the legs:
http://www.ebay.com/itm/NWT-baby-girl-pink-striped-swimsuit-/271293370817?pt=US_Baby_Toddler_Girls_Clothing&hash=item3f2a5851c1
So adorable, especially if there's some diaper peeking out as well!
Does anybody know of somebody that does custom cut swimwear? Or maybe somebody that does plus size swimsuit printing? I'm a size 32w (barely a D cup), so size may be an issue for some.