Here's a starter stats comparison with final stages/megas highlighted
My prediction for Mega Sceptile:
70 85 65 105 85 120
--- +30 +10 20 --- +40
70 115 75 125 85 160
(the site only talks about its speed, which was its main stat anyway, so in the same manner as Blazikens attack went up by whooping 40, this should be the case with Sceptiles speed. Other than that, I'm very certain it's physical attack will get the next biggest boost, to justify Leaf Blade being it's signature attack once again.)
Mega Swampert:
100 110 90 85 90 60
---- +50 +20 --- +20 +10
100 160 110 85 110 70
(Swampert has had amazing stats across the board since always. No matter how the 100 extra get distributed on top of it, it's only gonna get tougher and stronger. I'm expecting a huge +50 boost on its physical attack for obvious reasons, and at least a minor speed increase, as that will make a serious difference by making Swift Swim matter.)
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this is so much fun, I'll do one I'm sure we'll see too
Mega Metagross
80 135 130 95 90 70
--- +40 +30 --- +20 +10
80 175 160 95 110 80
I did a straightforward 40/30/20/10-mega distributed according to its already existing stats, while ignoring sp.atk.
Probably no broken ability as it's just as overkill as any other mega of a pseudo-legend already.
Since we got Alakazam and Gengar (although they messed up Alakazams stats, it only gains 90, because they probably increased the regular Alakazams sp.def by 10 after making the mega...), Machamp and Golem should follow!
Mega Golem
80 120 130 55 65 45
--- +20 +50 --- +30 ---
80 140 180 55 95 45
(the changes are similar to Aggron. And that's because I would also get rid of it's Ground type (Golem looks less Ground-ish than it's pre-evolutions anyway) and change it's ability to Solid Rock to turn it into the toughest rock ever, especially in a Sandstorm team.)
Mega Machamp
90 130 80 65 85 55
--- +40 +20 --- +20 +20
90 170 100 65 105 75
(obviously it's attack needs to go to monster level, but its speed would probably also get a noteworthy boost apart from its defenses. No clue what to make of its ability, No Guard is already great stuff though)
As Maxie and Archie clearly have Mega Stones on them, it probably means what many were already speculating, Megas for Camerupt and Sharpedo.
Which is actually nice, as they are pokemon that really kinda suck even for 460 TBS (until Sharpedo got Speed Boost that is), but probably won't evolve regularly again ever anyway.
Mega Sharpedo - Speed Boost
70 120 40 95 40 95
--- +40 --- +30 --- +30
70 160 40 125 40 125
Mega Camerupt - Adaptability
70 100 70 105 75 40
--- +20 +20 +50 +20 -10
70 120 90 155 95 30
They're both mixed attackers with awesome combos to go along, with Sharpedo being fast with barely any defense, and Camerupt being slow as hell instead.
I boosted both their attacks to similar standard-mega-levels, focusing on physical for Sharpedo and special on Camerupt, while adding some more speed to Sharpedo (its defenses aren't going anywhere useful either way), but making Camerupts defenses more noticable.
I wish there was an ability that would give Water immunity due to immensely high body-temperature, but the only thing that comes close is WaterAbsorb....so Adaptability is the next cool thing I figured (oh my. Double STAB on ground and fire moves sounds lethal).
It would be much more interesting for Sharpedo to get a different ability from it's HA, as that way you could Protect to boost at first, and then mega after you're done, changing to something even more overpowered, but seeing how that went for Blaziken I'm assuming they would do the same in this case.