Top 10 Mega Evolution that should have been
Sceptile and Swampert
Pokemon Omega Ruby and Alpha Sapphire (1)
These two are pretty much no brainers at this point considering they are the two remaining final forms for starters in the game alongside Blaziken. In X and Y, we actually didn't see the starting Pokemon get Mega Evolutions, but that was likely because they didn't feel new Pokemon deserved four stages yet. These two are established Pokemon that deserve to level up alongside Blaziken. With the ability to trade between games, it would be almost unfair for one starter to have the extra boost when the other two do not.
Swellow
Pokemon Omega Ruby and Alpha Sapphire (4)
Out of the six generations, we've only had two where the big bird evolution chain stopped with one evolution. The third generation was one of these with Tailow and Swellow, so I think it's about time we got another evolution in that line with Mega Swellow. We have yet to receive a basic flying type Mega Evolution, so this would be an excellent candidate as you will find it early in the game and can watch it grow throughout.
Sableye
Pokemon Omega Ruby and Alpha Sapphire (1)
Sableye is an example of a Pokemon that immediately caught my eye due to its really unique design, but also holds a really strong type advantage against Psychic Pokemon. There are very few Dark or Ghost type Mega Evolutions, and none with this dual type, so I feel it would be a good fit in the game.
Wailord
Pokemon Omega Ruby and Alpha Sapphire (3)
Wailord is already one of the largest Pokemon around, so who doesn't want to see him possibly get even larger with a new Mega form. Only two Pokemon in the original batch of Mega Evolutions had water as a type, and with the popular Blaziken around, water types are certainly a plus to have in Ruby & Sapphire. This is probably the least likely Pokemon on the list, but I've always been a fan of the giant whale Pokemon.
Kyogre and Groudon
Pokemon Omega Ruby and Alpha Sapphire (5)
Both Kyogre and Groudon are the new mascots essentially for Pokemon Omega Ruby and Alpha Sapphire, and the latest trailer teased possible Mega Evolutions for these two Pokemon. Combined with the fact that they are the two main legendary Pokemon in the game, we almost have to see them evolve to the next level in this game.
Rayquaza
Pokemon Omega Ruby and Alpha Sapphire (2)
The more questionable one of the legendary trio in the game is Rayquaza, the mascot for the third game, Emerald. We've yet to see a third game remake, so it would make sense to go ahead and upgrade Rayquaza here, if they plan on doing so in the future. However, they may choose not to upgrade every legendary in the game, and with Latias and Latios getting leaked for X & Y and likely Kyogre and Groudon, Rayquaza might not have as good of a chance. Regardless, I still think Rayquaza is one of the most deserving candidates in the game for a Mega Evolution.
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All Deoxys
Pokemon Omega Ruby and Alpha Sapphire (3)
While Psychic types were very prevalent in the last batch of Mega Evolutions and both Latias and Latios have this type, it still remains one of the most popular in the games. Due to this, if we are going to have an uber powerful new Psychic type Mega Evolution, Deoxys could be that Pokemon. However, the only reason why I didn't put Deoxys on the actual list is due to the Pokemon already having multiple forms within it's one level of evolution, which might be a little too tricky to manage in the game.
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