Part of the reason that I don't think there will be second is that why would they leave it out of the short that centered around the Eeveelutions? If there really is another Eeeveelution, they would probably want to market it, since a lot of people love Eevee and Eeveelutions, so it would draw attention, especially if it was Dragon, since people have been wanting a Dragon Eeveelution for a long time. The other part is that it would be overshadowed by Sylveon since it has a new type, and was actually marketed and promoted.Just like there is no proof that there could be another Eeveelution, there is also no proof that there won't be any.