Well, it's been awhile since I've done this, hasn't it?
Mega Clefable
Type: Fairy
Stats-
HP: 95
Atk: 70
Def: 73 -> 103 (+30)
SpAtk: 95 -> 115 (+20)
SpDef: 90 -> 130 (+40)
Speed: 60 -> 70 (+10)
Ability: Prankster
Move Additions (idk how many we get now so I'll put 1 for Clef and Machamp and 2 for Sudowoodo): Air Slash
Description: Upon Mega Evolution, Clefable's back wings grow larger and it becomes a bit chubbier. While these new, bigger wings don't allow Clefable to become a Flying-type, they do allow it to fly for a short time. Probably partially thanks to the energy emitted from mega evolving, Clefable uses it's new found wings to play playful pranks on trainers, including it's own, earning the ability Prankster. Along with gaining a great new ability in Prankster, giving priority to Clefable's wide array of status moves, Clefable gains a huge boost to Special Defense and Defense (because that's exactly what we want from Clefable), with smaller boosts to Special Attack and Speed, possibly because of the new wings, which also allows Clefable to now learns Air Slash.
Mega S U D O W O O D O
Type: Rock/Grass
Stats-
HP: 70
Atk: 100 -> 130 (+30)
Def: 115 -> 125 (+10)
SpAtk: 30
SpDef: 65 -> 105 (+40)
Speed: 30 -> 50 (+20)
Ability: Contrary
Move Additions: Superpower, Cut
Description: I'M STILL IN THE CUT, S U D O W O O D O.... Anywho, Sudowoodo, upon Mega Evolving, now actually becomes a tree, to an extent. It's still rock-like, but it now gains more leaves and even gains the ability to make it's own food via photoSYNTHESIS. But, now that it's an actual tree, that fact contradicts it's name, which is literally pseudo-wood. Well, yes, it does contradict it's name, and Sudowoodo embraces that by gaining Contrary. To take advantage of Contrary, Sudowoodo can now learn Superpower. As for Cut... have you been paying ANY attention since Pokemon Cypher 2016? Upon Mega Evolving, Sudowoodo gains a nice boost to Attack and Special Defense, patching that up real quick, as well as smaller boosts to Defense and Speed. I totally didn't give it a boost to speed to take advantage of Contrary Hammer Arm or anything...
Mega Machamp
Type: Fighting
Stats-
HP: 90
Atk: 130 -> 150 (+20)
Def: 80 -> 110 (+30)
SpAtk: 65
SpDef: 85 -> 115 (+30)
Speed: 55 -> 75 (+20)
Ability: No Guard
Move Additions: Mach Punch
Description: Machamp now goes from a supreme punching machine to a supreme punching machine that can actually outspeed some stuff! Upon Mega Evolving, the energy not only beefs up Machamp, of course raising it's Attack, Defense, and Special Defense a good amount, but Machamp is now a little lighter on it's feet, boosting it to decent levels, where if you have max speed, you might be able to confuse a Pokemon with the still-ever-present No Guard Dynamic Punch! Machamp can also now use Mach Punch (the thing can throw like 100 punches a second, how does it not learn Mach Punch?), surprising even more Pokemon with blazing fast punches!