I truly believe it's highly likely that the Pikachu line will gain a secondary Fairy typing.
Being in the Fairy egg group points the line in this direction, but there's also the fact that it's the Pokémon mascot that leans it towards this change.
Something small and cute that can dish out a beat down to fierce dragons; that's a hella cool poster child if you ask me. It attracts boy and girl audiences, being cute, and stronger than a dragon.
Not to mention, Pikachu has receieved a buff in every gen, making it more of a powerhouse. It was lightball buffs the last few gens, now it could be Fairy typing.
Also, XY has been described as an "evolution" of the series, so it is the perfect time to implement the change.

I mean, you could argue that they won't fix something that isn't broken, Pikachu is a really successful mascot already, but I think it's an opportunity to score big that Nintendo wouldn't pass up on.