Mega Evolution Expectations for Z

For the possible Mega Evolutions for Z, we need to look at specific patterns for Mega Evolutions, going from the likeliest to least likely:

Expected Megas:

1-Version Counterparts and Divergent Evolutions:

There have been two specific observations everybody immediately made, which was how certain Mega Evolutions in X/Y were version based. These were;

In X/Y:

Charizard X/Charizard Y

Mewtwo X/Mewtwo Y

Pinsir/Heracross

Tyranitar/Aggron

Latios/Latias

with Mawile, a former version exclusive, without its counterpart Sableye, and Gardevoir without its divergent evolution Gallade.

In OR/AS, we saw Sableye and Gallade get Mega Evolutions, and two Pokémon with divergent evolutions, Slowbro and Glalie, without their respective counterparts, alongside Beedrill, who has a version counterpart in the form of Butterfree. 

Another notable event was that a Pokémon with semi-unique evolution method, Scizor, have seen its evolution method brother Steelix see a Mega. Currently, the only known unique evolutionary methods in the Mega count are;

-Happiness evolution, by Lopunny.

-Trade evolution, by Alakazam and Gengar

-Dawn Stone evolution, by Gallade.

This gives us the following possible Megas;

Slowking:

Since Slowbro gained a Mega Evolution in OR/AS it is reasonable to expect that its divergent evolution will get a Mega as well. Since Slowbro and Slowking have similar abilities with inverse stats, it is reasonable to expect that Slowking's stats will look something like this;

Stats: 95/75/80/130/180/30

Ability: Battle Armor, which is known in Japanese as Kabuto Armor, or more correctly, Helmet Armor.

There are no reason to expect Slowking will get a particularly interesting set of capabilities, however.

Butterfree:

Since Beedrill gained a Mega Evolution in OR/AS, it is reasonable to expect that its version counterpart will do the same, especially considering the two continue to be version counterparts in Kalos games. As Butterfree and Beedrill's stats are relatively similar, it is reasonable to expect Butterfree will look like this;

Stats: 60/15/50/150/80/140

Ability: ???

As Beedrill didn't gain anything from Mega Evolution, it is reasonable to expect that Butterfree won't get anything either, but since Butterfree is incapable of learning Special Flying-type moves, we can expect a few different scenarios;

1)Butterfree will undergo a type change, presumably Bug/Psychic, to use its Sp. Atk and STAB bonus.

2)Butterfree, and likely by proxy Beedrill will get new moves depending on their Mega Evolutions.

3)Butterfree will get Air Cutter through Move Tutor, which was cut from OR/AS' final release.
Edit (06.09.2015): It was actually Roost Tutor that was cut out.

Froslass:

As Glalie gained a new Mega, one would expect Froslass to get one too, but that didn't happen. However, not only is Froslass Glalie's divergent evolution, it also shares the same method of evolution with Gallade, which makes it very likely to Mega Evolve. Since the two species are relatively similar, one can expect Froslass to look something like this;

Stats: 70/120/80/120/80/120

Ability: ???

Though both species benefited from the OR/AS expansion, just like Glalie didn't get anything for its new Ability, it is reasonable to expect that Froslass won't get anything either. One possibility to consider is the fact that it might gain Fairy sub-typing, as it now learns Draining Kiss, but that's a high shot.

2-The Recurring Roles:

In addition to above, it is well shown that Pokémon which fit specific roles in-game are more likely to get Mega Evolutions than others. These are;

-Starter Pokémon, as shown with Kanto and Hoenn starters.

-Regional Early Route Electric-type, as shown with Ampharos and Manectric.

-Regional pseudo-legendary, as shown with Garchomp, Tyranitar, Salamence and Metagross.

This gives us the following list of Mega Evolutions for Kalos;

1. Chesnaught
2. Delphox
3. Greninja
4. Heliolisk
5. Goodra

3-Everything else:

Outside of this, it is known that Mega Flygon was intended for OR/AS, but didn't make it. We have seen that the line got several new moves, mostly sound based ones such as Uproar, Bug Buzz and Boomburst as well as Dragon Rush, so it is reasonable to expect that it will be special based, possibly a Bug/Dragon.

What else to expect:

We also know there are other factors when it comes to what Mega Evolutions are chosen. These lesser factors, or rather biases, are as follows:

-In each release, there is at least one species from each generation. In X/Y, we had Megas from each generation except Gen V and VI. In OR/AS, we had Megas from each generation. It is likely Z will continue OR/AS' route.

-If a species is a two stage Pokémon, or a one stage Pokémon, it is more likely to get a Mega above others. This is seen with the bulk of Mega Evolutions, such as Banette, Lucario, Audino, Sableye etc.

-If it is a three stage species, it is likely that it will be selected from the list of Pokémon whose BSTs got updated. This is seen with Alakazam, Ampharos, Pidgeot and Beedrill.

-In each release, there is at least one type represented. In the current situation we have the following types "represented";

1. Bug
2. Flying
3. Ground
4. Dragon
5. Ice
6. Ghost
7. Water
8. Psychic
9. Dark
10. Grass
11. Fighting
12. Electric
13. Normal
14. Fire

This gives us;

1. Poison
2. Steel
3. Rock
4. Fairy

As "remaining".

-The Pokémon who were formerly trademarked for merchandising will get Mega Evolutions. This is already well known in the fandom, so there is no point in repeating it outside of linking the list for pre-Gen VI trademarks and Gen VI trademarks.

 For Kanto:

For Kanto, we have two species who fit this roles, these are;

Wigglytuff and Clefable:

Not only the two species have been trademarked, Wigglytuff's trademark continuing until 2019 and Clefable  trademarked by another company, both species have also gained a BST update and both species cover the necessary Fairy-type roles.

The only notable obstacle on Wigglytuff's path is the fact that it recently gained a new Ability, and so far no Pokémon with Ability update gained a new Mega Evolution.

For Johto:

Unknown. Currently the only notable trademarked Pokémon are starters and legendaries, and the only important Pokémon that are trademarked by another company are Donphan and Octillery.

For Hoenn:

For Hoenn, we have only one species that fit these criteria;

Exploud:

Having gained a BST boost, Exploud was trademarked for merchandising in 2003, which expired in 2013. It is very likely that they will have another shot with it.

For Sinnoh:

As Sinnoh has a good list of trademarked Pokémon that are two stagers or single stagers, we will look for the BST updated Pokémon, which is;

Roserade:

Not only Roserade has a trademark in the line, immense popularity and a BST update, it also has a nasty wide movepool that allows it fill several different roles.

For Unova:

For Unova we have two different Pokémon that fit criteria;

Gigalith & Krookodile:

These two not only have trademarks, they also have BST updates.

What else is there:

Machamp:

Not for the above listed, but since it is one of the few Pokémon from Pokkén Tournament that doesn't have a Mega and one of the Kanto trade evolutions to do so, it is likely to get a Mega.

Weavile:

Same as above, but Weavile actually has a trademark left from the Lucario & Mew movie.

Jynx:

One of the Pokémon that was trademarked alongside the original Mega set, Jynx is still waiting to get a Mega.

Other trademarked  Pokémon are also very likely, as always. So are the Megas for in-game purposes, such as villain Pokémon, E-4 Pokémon and the rival's Pokémon(though this might be fixed by the Mega Starters).

What not to expect:

Counterparts of Pokémon that are listed here; such as;

-Politoed and by proxy Poliwrath, as we haven't got Slowking's Mega yet and Politoed didn't get the BST Poliwrath had.

-Other early Bug-types, especially Beautifly and Dustox, as Dustox didn't gain the same BST update as Beautifly, but also because the archetype hasn't really developed into the one of the main archetypes that is guaranteed to have a Mega. Same holds for other early birds, and other late-game Ice-types*

Expected Pokémon for future remakes of later generations, such as:

-Starters, pseudo-legendaries and early electric-type Pokémon of Sinnoh and Hoenn.

Double battle designed Pokémon:

-Illumise & Volbeat, Solrock & Lunatone, Plusle & Minun etc. Though these all have trademarks, since a team can only have one Mega, it is impossible to make them into Megas while keeping their double battle roles. The possible exception to this might be Meowstic-F and Meowstic-M, who actually have a hidden super Psychic power.

Forme based and Pokémon with radical gender differences Pokémon:

-Not because of logical impossibility, but because it hasn't happened yet. Outside of the above exception, and possibly Pyroar and Furfrou of course, which might get Megas for other reasons, thus killing this criteria.

*Though this might actually have developed into one of the few archetypes that is recurring. In this case, Mega Avalugg and Lapras would be likely.