Dragon & Ghost are the perfect match in my opinion! I approve ^^Alright, I'm bored, so here's my take on what Eeveelutions I want to see if Gamefreak goes that route:
Dragon is the obvious choice, due to it being the last (formerly) special type, bit it actually works well- not only will a dragon -eon give players the more averagely-powered dragon-type they've been wanting, but since it's such a distinct type it'll work well if this generation introduces only one eon. But if it does introduce a pair, I think dragon would go well with...
Ghost. Not only would it complete the "se-on-self" set, ghost is a rare and enigmatic type itself.
Barring that, steel and poison would complete the "resists self" set of types for Eeveelutions, and I also feel like they go well together in some vague, abstract way.
And finally, flying, because it would be the perfect excuse for a mundane pure-flying type, which as you all know I totally want.
... actually, heck with it Gamefreak. Do all of these. Five new Eeveelutions in one gen. Yeah.