Looking for ideas for a quilting motif or pattern for this one. I'm thinking something to echo the diamond shapes? It is 86"x68" and will be quilted on a DSM.

I would do a meander or large stipple.

Beautiful quilt.

If you're thinking diamonds, a simple diagonal crosshatch would work. I would vary the distance between the rows of stitching to keep it interesting.

Beautiful quilt and I love the colours. IMHO this diagonal crosshatch sounds interesting and I think it will work well. Thank you for sharing.

A loopy meander throughout the center in the center with a bit more tight FMQ on the borders might look very nice.

Because the quilt is so geometric, I would se a meander.

I love the quilt. I would do a meander too, because there is enough to look at just looking at the piecing. I think too much quilting would conflict.

I would straight stitch on each side of the diagonal blocks, then on each side of the internal squares. Save your FMQ for the borders, as others have wisely suggested. Love your colors.

I would do three lines of diagonal crosshatching, up the edges and middle of the "ladder" part.
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You will need to do more quilting inside the squares; you could use any curvy fill in there (meander, paisley, loops, etc), or you could quilt in a motif like a feathered wreath or a leaf or something. I would choose something that suits the recipient.

For the border you could do straight lines parallel to the edge, or gentle waves, or a meander or fill to match the squares.

I like the subtlety of the pattern with your colours. It feels very natural. Good luck with the quilting!