With 20+ mega evos confirmed and given how spread out they are, I couldn't help but notice the elegant pattern.

Have a run down through all of the mega evos.

How many of the pokemon eligible for megatransformation require an evolution-inducing item that isn't linked with evolve-upon-trading?

ZERO.

Therefore, if a pokemon requires an evo stone/claw/scale/whatever, it not only won't receive a mega in XY, but in the upcoming generations as well.