It may end up being Sylveon that replaces Lucario, but I kind of hope not.
Something that I would see as cool is having Reshiram and Zekrom. They could have some kind of gimmick about their switching, I'm not sure yet though. But Reshiram could have long range attacks and be faster, while Zekrom could have short range attacks, and be slower, but stronger. Final Smash could be Kyurem.
That being said, it wouldn't happen. They aren't going to advertise gen 5 Pokemon when they could advertise gen 6. Plus, the whining about gen 5 Pokemon would become insatiable.
It wouldn't be Sylveon...
It'll be something that's like Lucario/Zoroark.
it'll probably be one of the next few pokemon they announce