Gigalith
Rock ----> Rock/Psychic
Sturdy, Sand Force (Hidden) ----> Photoluminescence (When Gigalith is hit by a special move, its Special Attack is doubled until the end of the next turn. This boost does not stack or refresh, and is lost if Gigalith switches out.)
85/135/130/60/80/25 ---> 85/125 (-10)/140 (+10)/125 (+65)/80/60 (+35)
New Moves: Trick Room, Psychic, Dazzling Gleam
Concept: Gigalith is always fun to use on Trick Room teams, so I decided to go the "energy = Psychic" route and give it the move. Mega Gigalith forces the opponent to either give it the one-turn special attack boost, or try and work through its physical defense, or hit it on the special side hard enough that it can't KO something.
It has plenty of coverage options: STAB Power Gem and Psychic, and then Dazzling Gleam, Earth Power, and Flash Cannon. Power Gem and Earth Power are near-unresisted, and any of its other moves will fill in all the gaps there (except Psychic for Claydol, but that's not really an issue). Psychic might not see much use, but it's there. Dazzling Gleam hits Dark types hard. Gigalith's attack is lower after its mega evolution, but it's still high enough to hurt something like Blissey with Superpower or Explosion.
60 is kind of a risky speed for Trick Room, since at min speed Mega Gigalith is outslowed by base 36s and lower even when they're 31 IV + neutral nature, and some of them can take even a boosted attack from Gigalith and 2HKO it. If Gigalith is faster than something with a special attack (or if that something can't hit hard enough to 2HKO Gigalith) then an already-mega-evolved Gigalith can come in on the attack and do some serious damage even without Trick Room.
A faster + specially based opponent can use Trick Room to its advantage with Gigalith, avoiding a boosted attack by hitting it with a special move after it has already moved, and then avoiding the boost's effects on the next turn with Protect, Prankster Substitute, switching to a resist or immunity, etc. Gigalith's lack of recovery means it can't keep taking special attacks, especially from its weaknesses (of which it has quite a few). It would do well in sand, though.