Mega Diancie has been confirmed in a CoroCoro scan. Diancie gains the magic bounce ability after mega evolving, as well as gaining a bump in its stats such as Speed, Special Attack and Attack, however, its Defense and Special Defense both receive a reduction.
What does Magic Bounce (ability) do? – This ability reflects the effect of entry hazard moves (such as stealth rocks), stat lowering status moves (besides Memento), non-damaging status condition-inducing moves (such as sleep powder), and moves such as Taunt and Torment.