That's not really fancy unusual wing design. It's just Flandre with butterfly wings.

it is unusual because it's not something you would expect to see on Flandre its very different from her normal wing design so it should go in the pool ,but i guess ill wait for other people to voice opinions before we keep up with switching in and out of the pool.

Also you could say that "it's just Flandre with _____ wings." for any pic in the pool, like i said it's more that it's not something Flandre's wings are usually shown as so it should go in the pool.

And that's why it is already tagged with alternate_wings.

I don't see why the two have to be mutually exclusive, it fits both the tag and the pool so why can't it be in both?

Because the pool would then be redundant. The pool itself says "most unusual wings designs" and not just any wing design other than the normal one.