D5100 - add on suggestion please

My current setup is

Nikon D5100
Nikon 18-200 f/3.5-5.6 VRII
Nikon 35 f/1.8
Popup Flash Diffuser & Kobre Lightscoop

I am thinking of upgrading or adding to my existing gear. I shoot a combination of family portraits & events, travel and some nature. Macro photography is something I want to try at some point of time.

With a budget of, say $750, what will be the most logical or best purchase be? Some of the things that came to my mind are

Nikon SB-700 flash (I don't have any now)?
Nikon or Sigma or Tamron macro lens?
Sigma 17-50 f/2.8 or another constant aperture lens?
Replace 18-200 with 16-85 or 18-105 & 70-300?

I understand none of these fall in one category but these are just options I could think of. If you have other suggestions, thats great too. Please help!