My predictions for Mega stat changes:
Salamence:
HP: 95
Atk: 135 > 155 (+20)
Def: 80 > 120 (+40)
Sp. Atk: 110 > 120 (+10)
Sp. Def: 80 > 100 (+20)
Speed: 100 > 110 (+10)
It already has amazing Attack and good Speed, plus it has access to Dragon Dance, making said stats even better @_@ I could maybe see the increases in Sp. Atk. and Sp. Def. being swapped (making it 130 Sp. Atk. and 90 Sp. Def.).
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Altaria:
HP: 75
Atk: 70 > 120 (+50) 70 > 100 (+30)
Def: 90 > 100 (+10) 90 > 110 (+20)
Sp. Atk: 70 > 90 (+20) 70 > 100 (+30)
Sp. Def: 105 > 115 (+10) 105 > 105 (+0)
Speed: 80 > 90 (+10)
70 base Attack is rather low, so I can see it getting a big boost. 90 Speed, while not the greatest, isn't too bad when you consider Altaria has pretty good defenses (and access to Cotton Guard to further boost its Defense). Plus, like Salamence it also gets Dragon Dance, so it can boost its Speed if need be.
I feel like Altaria might not get a boost in Special Defense since it already has it pretty good in that stat as is, and it seems like its Attack and Special Attack would have the same total.
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Lopunny:
HP: 65
Atk: 76 > 106 (+30) 76 > 116 (+40)
Def: 84 > 94 (+10)
Sp. Atk: 54 > 94 (+40) 54 > 64 (+10)
Sp. Def: 96 > 106 (+10)
Speed: 105 > 115 (+10) 105 > 135 (+30)
I didn't realize how bad most of Lopunny's stats (particularly its attacking ones) were until I looked it up @_@ Anyway, since it was stated it'd receive a boost in Attack and Special Attack, I imagine the other stats won't receive too huge a boost, especially if they want to make Lopunny somewhat mixed.
Since Lopunny's apparently getting a boost in Speed and NOT Special Attack, then I have a feeling things might be switched around, with its Attack going up by 40 points to make it stronger and its Speed by 30 to make it pretty fast.