Not sure how to quilt this once I get it all sandwiched... any ideas? I particularly wanted to play with a contrasting colored thread design in the dark blue "trellis lines"....

It is a beautiful quilt. However you quilt it will look nice.

My first thought is a variegated thread with flowers/vines in a random design all over ... just as vines would twine all over a real trellis.

Though, would you please show us a picture of the whole top, and not just segments? Also, how large is each diamond? and the lattice strip widths?

It might give us a better sense of what you have.
Thanks!!