Its always felt like a flying type or normal type to me, and with eevee being a normal type already, I'm saying Sylveon is flying type. Plus I've always thought a new normal eeveelution would end up looking a lot more like a grown up eevee than what we've got with Sylveon...
Based on the design, which you have to admit is quite feminine, I'd say there will be another eeveelution in gen 6 with a more masculine design. This could cover any of the remaining types not used (rock, ground, poison, steel, bug, etc), and would be a new method of evolving eevee... Maybe when it reaches a specific level based on gender it evolves into the new eeveelution? Or maybe there's an area in the new region that promotes feminity, and one that promotes masculinity... like the Pokemon Musicals and Challenger Rock in Unova?