I spoke of Mega Magnezone earlier. I cannot speak of any other without first mentioning that divinity.
Now that I have paid homage to the one true Magnet God, I may now proceed to theorize about other, lesser Megas. For instance:
MEGA ROSERADE
Type: Grass/Poison (unchanged)
Ability: Prankster
HP: 60
Atk: 70
Def: 85 (+20)
SpAtk: 155 (+30)
SpDef: 135 (+30)
Speed: 110 (+20)
Roserade is one of those Gen4 last-stage evolutions, like Yanmega, Togekiss and MAGNEZONE that deserves a Mega to remain relevant in a changing metagame.
Roserade's primary role as of now in UU is to act as a Spikes/TSpikes lead with access to Sleep Powder and a good defensive typing that lets it take on water and fighting types, or as a Powerful special attacker with excellent moves like Leaf Storm, Giga Drain, Sludge Bomb, Extrasensory, Shadow Ball, Dazzling Gleam and a choice of Hidden Power to use alongside status or hazards. It has a good base 125 SpAtk stat and decent 105 SpDef stat along with Synthesis and Natural Cure Rest to use with this, but is let down by poor physical bulk and rather average speed.
Mega Roserade reclaims its DPPt position with relative stat modifications. An increased Special Attack and Special Defense bolster its good points to current levels of power, and a significant boost to its pitiful Defense stat lets it take at least the few odd hits.
A good speed boost puts on par with other powerful offensive mons like the Lati twins, Gengar, M-Metagross, M-Gallade (and M-Magnezone.) So far, Mega Roserade is like Mega Scizor: an updation of an existing model with changes in degree not in kind.
The real change is in giving it Prankster, to let it play shenanigans with its great support movepool. If you thought Whimsicott was bad, this is version 2.0 with good bulk and great power! Prankster Hazards and Substitute! Prankster Sleep and Paralysis and Poison! Prankster Recovery with Synthesis and Leech Seed! It can even use Prankster Sunny Day and act as a Sun starter whilst grabbing itself a pseudo-STAB in Weather Ball. The creativity is endless.
However, it isn't broken by any means. It is still checked by many common Steel and Fire types if it can't sleep them or if it hasn't got HP fire/ground, and it is still Revenge killed easily enough. The 110 speed tier is also crowded enough to prevent it from being too easy to use.