IF a new type is introduced then I think it would be light.
Sylvleon;
However as for now it is looking like a flying type as it has been put with Jolteon, and eeveelutions are always put with their (dis)advantage types such as Glaceon and Flareon.
So it could either be FLYING or GROUND.
Its design is more on flying than ground.
Legendaries;
AND if a new type is introduced then that being light. Then I think Xerneas would be a light type and Yveltal will be dark.
EVERYONE LOOK!
http://25.media.tumblr.com/60c644da00b2c6ced2dba3f9db9a92d1/tumblr_mj2et4gnjs1rurq6go1_1280.jpg
ALSO SHOWS THAT SYLVEON IS A NORMAL TYPE,
HERRACROSS AND PINSI