A Ground one also fits in the gaps. Glaceon and Flareon both are SE against Leafeon. At the same time Leafeon and Jolteon are SE against Vaporeon. A Ground Eevee fits perfectly to create circles.
The long-awaited Dragon-type (Dragon being the last pure-special prior to the split) is probably next, since they just did Fairy, which is immune to Dragon. Trollfreak would do it. No clue on names.