well, if you think about it, Umbreon was one of two new types introduced in it's generation, this being Steel and Dark. Espeon represented the sun, and Umbreon the moon.
Sylveon, if correct, would be this new type, while the other "counterpart" would be an existing type. If Sylveon looks as if it represents life(Fairy/Light), then its counterpart would/might represent death(Ghost) or artificial life(Cyber). But these are just my simpleton thoughts about implementing a new type.
Although your right that Cyber is kinda familiar, because it can either be Electric if your going with "data", and Steel if your going with "machines".
So, I guess we "should" avoid using types not confirmed yet, but you can still change the type when it "is" confirmed.