I remember someone once making the point that Eeveelutions tend to be based on "Special" typings. Prior to the Physical/Special split in Gen 4, all Eeveelutions were based on types that were Special.
With that theory now thrown out the window, I'd love to see some more Eeveelutions. I loved that they were introduced in pairs, with Espeon/Umbreon and Leafeon/Glaceon. I guess Sylveon was made to help promote the Fairy type (which has definitely gone far enough lol), so it makes sense that there was no pair there.
But I'd love to see two more. Perhaps Ghost and Dragon?