So I know I'll get shit for speculating, and half of you will ask if I'm John Gamefreak or something to that effect, but oh well.
Here are my prediction for ORAS megas based on GF's obvious subtle hints in previous official information, in order of likelihood from greatest to least.
Know anything I missed? Feel free to post your theories.
Note that some of these also have a presence in the new demo, but having not played it myself, I won't use that as a source yet.


Shiftry
----------
Shown in battle against Mega Gallade, Mega Altaria, and Mega Beedrill at http://www.pokemonrubysapphire.com/en-us/pokemon/new-mega-evolutions/new-mega-evolutions*
Shown in battle against Mega Beedrill at https://www.youtube.com/watch?v=RFvxSvPIw5c**
Shown in battle against Mega Gallade at https://www.youtube.com/watch?v=HuaL_hMpPaM***
Shown in battle against Mega Altaria at https://www.youtube.com/watch?v=hFZuWPXPcyI****
Shown in battle against Mega Blaziken at https://www.youtube.com/watch?v=P-wTDPviFq8


Duskclops
----------
Shown in battle against Mega Diancie and Mega Sharpedo at http://www.pokemonrubysapphire.com/en-us/pokemon/new-mega-evolutions/new-mega-evolutions*
Shown in battle aginst Mega Sharpedo at https://www.youtube.com/watch?v=HuaL_hMpPaM***
Shown in battle against Mega Sceptile at https://www.youtube.com/watch?v=P-wTDPviFq8

continued in next post...