We need and eeveelution for every type, every type combo, and each eeveelution has a Mega.
JK but seriously, they usually introduce eeveelutions in pairs, with one having the type advantage over the other, but Sylveon was introduced solo, with no counterpart, so I would say dragon, or ghost(Since ghost types are cool)
Turns out an eeveelution for every type and type combo would give us atleast 171 unique eeveelutions :O (Bulbapedia says there are 171 unique type combos)
We would need a separate Eevee dex!