Welcome to Gaia! ::

Here's a couple ideas to get things rolling.

Primal Regigigas

Required held item: Royal Orb (A jewel that once belonged to a king, sealed away for fear of its awesome power)
When the king of the legendary golems returns to his ancient form, his Defense, Special Attack, and Special Defense all increase. He also gets a new ability, King's Fury: When this Pokémon is affected by poison, burn, or paralysis, Attack and Speed are increased 50%. Stat drops from burn and paralysis are also ignored.

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(Alternate plan 'B' for Regigigas)
Breaking news! After years of research, scientists have discovered that the DNA Splicers, once thought to work exclusively on Kyurem, can also combine the genes of Regigigas with certain Pokémon.

Combine with Regice:
Gains boosted Special Attack and Special Defense
Ability becomes Clear Body

Combine with Regirock:
Gains boosted Attack and Defense
Ability becomes Rock Head

Combine with Registeel:
Gains boosted Defense and Special Defense
Ability becomes Sheer Force

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Mega Snorlax

Mega Stone: Snorlaxite
By Mega Evolving, Snorlax gains boosted Attack, Defense, and Special Defense, but loses a little speed. It also gets a new ability, Solid Fat. Moves that make direct contact have their damage reduced by 33%.

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Mega Jynx

Mega Stone: Jynxite
She gains a much needed defense boost, plus stronger special attack and speed. I wanted to make her an Ice/Fairy-type, but her movepool just doesn't support the type change. Maybe Gen VII can fix that, but for now I'll leave her Ice/Psychic. Her ability could be something like Serene Grace, giving things like Ice Beam a better freeze rate.