
Bellossom
Typing: Grass -----> Grass/Fairy
Abilities: Chlorophyll / Healer -----> Pollen Shield (Raises defense by 50% in strong sunlight)
75/80/95/90/100/50 -----> 75/105/120/110/125/55
Moves: Quiver Dance
Fanon Pokedex Entry: Mega-Bellossom's petals become more vivid in the sunlight. When it is happy, it performs an elegant dance and releases a pollen that soothes those around it.
Alright, so Bellossom becomes a monster wall under the sun. Pollen Shield is like Flower Gift but only defensive. Bellossom gets Quiver Dance for the same reason Lilligant does; even after a boost, though, it's low Speed still enables it to be outsped by faster or Scarf'd opponents. 110 Special Attack isn't amazing, especially for a Mega, so Bellossom needs to boost to be useful. Basically, Bellossom switches in under sun and tanks anything that isn't super-effective, sets up with Quiver Dance, and then attempts a sweep. It's not perfect. Grass is a shit offensive type. Grass/Fairy isn't a good defensive type (not the worst, but still). Fire- and Flying-type weaknesses (the former even worse under sun). 55 Speed with a 2-stage boost is still outspeedable by quite a few threats. It kind of sucks out of sun. On the other hand...: 75/180/187 bulk under sun (...mother of God...). Quiver Dance. Fairy-STAB. Ability to tank common Water-, Fighting-, and Dragon-type moves. STAB Solar Beam in the sun off of a 110 Special Attack boosted 2 stages by Quiver Dance. Really, that's all you need.
TL;DR: Ungodly tanky under sun, mediocre without. If you can keep the sun up this will be a worthwhile Mega. It needs to Quiver Dance at least once to be useful (the temporary extra bulk helps), but even after it pulls one off, 110 Speed isn't terribly fast (Greninja, Tornadus, Thundurus, Talonflame...). On a sun team based around it, it can be very useful, but certainly not unstoppable.
If you notice, no more huge paragraphs. I don't have the time ATM, but let's see how it affects my submissions.