1. The fact that Yuuki was willing to risk her life in order to provide Zero with blood, even with her phobia of vampires at the time, seems to me to be proof that she loved him even before she was aware of this fact.
2. The fact that, with what they both are, they really ought to be enemies but circumstances led to them falling in love instead. That seems "meant to be" of "fated to be", to me.
3. A pureblood vampire with a vampire hunter/ex-human vampire really fits the pacifism and co-existence theme of the story.
4. The fact that Zero's blood alone seems to have sated Yuuki for a thousand...