Ficus grafting opinions.

What's up guys, I picked up this hunk of wood for a spring project to help fill my time.  It's a ~6'(1.8m or so) tall braided Ficus benjamina that's roughly 35 years old.  I decided that it would be an excellent foray into advanced techniques such as approach grafting and that I would like to graft on another species of ficus since benjamina is generally not well looked upon.
I was wondering if anyone had suggestions regarding what species would make for a good graft, or potential graft placement and style ideas.  I like the vibrant leaves of microcarpa varieties, but have had trouble with filling out inner branches on my 'green island' project tree.  I had thought that F. Triangularis would be interesting since it has an unusual leaf shape and fruits, but I also know that the small leaf variety of F. Retusa is very popular.  
Alternatively, since I would like to do this project as a sort of strangler(ish) style to help cover over areas where the tree has less girth than it should, would I be better to latch onto species like F. natalensis or F. phillipensis which tend to throw out aerial roots quickly?

This is sort of a rough, rudimentary, preliminary virt of what I was thinking style wise. Using the trunks of grafting material to fill in spaces where the current trunk is lacking girth until enough roots could be grown and fused to make a solid, decent looking trunk in the years to come.