I quite agree, the idea behind this is brilliant. And as you said the execution is just "Superb".
Usually when you see Flutteshy/Discord pieces, it usually involves happy friendshippyness, Fluttershy embarrassing Discord or Discord trolling Flutters, which are all fine by themselves. But the idea behind this seems more fresh and original, at least to me.
What makes this image great, I feel, is that we get to see Fluttershy and Discord being a bit different from their usual selves, but still in a way that makes them work off of one another's characters. I.e. Fluttershy, gets to be a confident beauty, as opposed to a shy beauty. And Discord gets to be vicious, focused and powerful, as opposed to all over the place, random and chaotic.
Put together, they get this "unique spark" of which you speak, Forest-Lark. A spark that is powerful and tense; and is in juxtaposition to the usual light-hearted, warm and happy ambiance such Discord/Fluttershy pieces usually convey.
Personally, I would love to see many more pieces exploring this compelling motif.
I love this concept This is my favorite version of their relationship, of her as some kind of conscience as he tries to control his desires of ending with the world we know. Amazing pic by the way, that serene Fluttershy and furious Discord are simply perfect.