Hi Renata -
I'm still learning about how to look at a quilt and pick a quilting design that complements your work. I think your block is just beautiful!! If I were to make a quilt with just this block and no borders, I think I would first SID as suggested. I would probably also stitch around the flower as best I could to make it stand out, maybe with some stippling in the black background. This would be really nice with a wool batting that would make the flower puff out. Since I'm not a fan of invisible thread I would probably use black thread, but invisible thread would be completely appropriate here if you like working with it. For me it's hard because I can't see the lines I've quilted while I'm working so it ends up quite messy, although no one can see that.
I would then think about some sort of curvy black line in the white ring. Maybe a string of pearls or a string of curly-Q's. I really love the white frame, but think some black lines in there would tone it down a bit and bring everything together. I'm sure others have some much better suggestions, but those are my first thoughts. And if you want to add a border, I would add that and then look at it some more before making decisions, just in case.
Nancy