Anything they add into these new games can be patched into XY and ORAS can just block trading with XY games that are not a certain version number. Very, very easy to pull off for a first-party developer. The only reason we didn't have patches in previous gens is that it wasn't possible to do with a Gameboy or DS.
Sugimori or someone from GF actually stated they were thinking rebalancing Parental Bond since VGCs made it pretty obviously broken. I wouldn't count out anything from them yet.
I'm pretty sure a patch will be released to add these new Mega evolutions into X and Y, although I'm doubtful new Mega evolutions apart from Mega Latias and Latios will be added in Ruby and Sapphire.
I doubt Sceptile and Swampert will be left out in the cold nor will Blazikenite remain an event item. Hell, I wouldn't be too shocked if Mega Lati@s see a redesign since their XY models can be patched-out and replaced. Nothing's really set in stone anymore and they Mega Lati@s we know could just be placeholders or a failed idea that wasn't removed; still not accessible without save editing either.
It wouldn't be the first time beta elements have been left in.
But what I'm more interested in than starter Megas are Move Tutors. Like allow me to just make my Christmas list here...
Knock Off Greninja
Knock Off + Icicle Crash Weavile
Defog Poison Heal Gliscor
Parting Shot on a half-decent Pokemon (Furfrou would be enough)
Freeze Dry Kyurem
Stealth Rock Klefki
After You Klefki for Doubles ("No I insist, Ms. Kangaskhan, YOU go first this turn")
And while not tutor related, Contrary Serperior and Adaptability Dragalge when?
It's not even E3 yet and Nintendo is dropping bombs. Please be gentle, Iwata-kun~