I'm a fan of sailor tattoos and I wouldn't go with a web either unless it was behind the rose and obvious what it was. It might take attention away from the swallows. What about the thorns idea but going under the birds in the same style as the roses the birds hold? Or if you were going to get butterflys extending them to underneath your coller bones behind the birds towards the edge of your shoulders if you get what I mean. I like the space inbetween the birds and the rose It looks defined I think extending it is a good idea but filling in gaps might make it look more square and less of a V shape if all of this makes any sense at all.
Katx