I'm not a fan of stippling, so I think I would have fun with it. I wouldn't want to detract from the lovely embroidery, so I would use very pale thread, possibly white. I would probably outline close to the embroidery, not on it. In the block with the girl on a swing I would quilt a tree to the right, some grass, a bush to the left. On the dog, a cat where the girl and dog are looking, grass, maybe some bushes to fill in the space. On the one with a net, some butterflies. You get the idea. I would do something with straight lines in the sashing, plus SITD. I think you have a beautiful quilt with lots of possibilities.