Timid Rotom-Frost with SpAtk, SpDef, and Speed IVs.
Task: Mitigate Barbaracle's 4x Grass Weakness.

Defensive set or Offensive set? EV spread? Would 31s in HP and Defense and a different nature be needed for different stuff?

These were listed on the leak stuff from reddit. Take with a grain of salt. While some of them would be nice, some of them just don't look believable.
Mega Meganium, Grass/Dragon, Regenerator
Mega Typhlosion, Fire/Fighting, No Guard
Mega Feraligatr, Water, Strong Jaw
Mega Fearow, Flying/Dark, new ability that is like pixelate for dark types
Mega Butterfree, Bug/Psychic, Levitate
Mega Nidoking, Poison/Ground, Moxie
Mega Nidoqueen, Poison/Ground, Regenerator
Mega Jynx, Ice/Psychic, Simple
Mega Mr. Mime, Psychic/Fairy, Prankster
Mega Slowking, Water/Psychic, Magic Guard
Mega Donphan, Ground, Huge Power(no atk boost outside of ability, but defense and speed are increased)
Mega Dunsparce, Ground/Fairy with Serene Grace, gets Play Rough via move tutors
Mega Xatu, Psychic/Flying, Magic Bounce
Mega Flygon, Dragon/Ground, Levitate
Mega Delcatty, Normal/Fairy, Pixilate
Mega Exploud, Normal, Scrappy
Mega Milotic, Water/Fairy, Pixilate
Mega Luxray, Electric/Dark, Strong Jaw
Mega Vespiquen, Bug/Flying, Aerilate
Mega Weavile, Dark/Ice, Serene Grace
Mega Electivire, Electric/Fighting, Motor Drive
Mega Magmortar, Fire/Psychic, Flame Body

There's also a rumor that players will be given a Johto Starter at some point in Alpha/Omega. It would make sense (its similar to X&Y and would justify more new Mega Stones) but it still disappoints me.

The Johto Starters are just so boring...

Well i'd expect them to give you Johto starters at some point, i mean they did that in Emerald.

The rumoured Megas list is beginning to look a little TOO big for this game now to be believable...

Jynx? Pokemon has been trying to forget they made Jynx after the blackface BS.
Also, the list is getting way too big now.

Also, the Johto starters are awesome~
http://i.imgur.com/7ZRBgho.gif
Or at least Totodile is~

Jynx most certainly should be getting one along with Plusle and Minun. They were on a list of copyrighted names and all the other names on the list already have Mega Evolutions.

Curious~

http://pokemonfigure.blogspot.jp/201...a-pokemon.html
This one has Jynx listed. Plusle and Minun were on another list that had a couple of Pokemon who've gotten Mega Evolutions in ORAS so far like Rayquaza and Sceptile. Groudon and Kyogre were also on the list.

Ah, neat~