Mega Pidgeot, which has No Guard, meaning its attacks will always hit, regardless of accuracy. However, it will also always get hit by attacks, regardless of accuracy.
Mega Beedrill, which has Adaptability, meaning its Poison- and Bug-type moves are powered up.
There are now some islands and shit where you can get non-Hoenn legendaries.
Then there's Mega Latios and Mega Latias. You get them the same way as in normal RSE.
I've got to say, Mega Beedrill is looking pretty fucking sweet.