Normal / Fighting
Att: 175 (+60)
Def: 75 (+15)
SpA: 70 (+10)
SpD: 75 (+15)
The ability flavor is a corruption on the original idiom, "seeing the world through rose-tinted lenses". A more straightforward way of describing the idiom literally would be "seeing red", which is itself an idiom for being extremely pissed at something on sight. Mega Zangoose is going to click Close Combat, and if it sees some dirty stinkin' no-good piece-of-shit Poison-type thinking it doesn't have to care because it resists Fighting, Mega Zangoose is going to funnel its rage into beating that Poison-type's smug face into a bloody pulp. Its eyes are also literally red, and one of them has a red fur streak going through it.
A Normal/Fighting Pokemon with Tinted Lens, 175 Attack, Swords Dance, and Quick Attack may seem like it'd be busted, but you need to keep in mind that base Zangoose has similarly insane neutral coverage with Facade, is even stronger than Mega Zangoose thanks to Toxic Boost, and is still a mediocre Pokemon. Mega Zangoose trades some of Zangoose's explosive power for a stronger hit on Steel-types, but its better longevity is likely the biggest advantage Mega Zangoose has over Zangoose. It's still pretty frail despite the boosted defenses, but the lack of a constantly-ticking Toxic clock and the added resistance to Stealth Rock drastically improves Mega Zangoose's staying power in the face of passive damage. That said, it performs poorly in the face of direct damage. Quick Attack means it's not helpless
against offensive teams, but between its frailty, middling Speed, and Close Combat drops, it still struggles.
It's also worth noting that Mega Zangoose has an additional moveslot to play with compared to Zangoose. Zangoose ran Facade for its strong neutral coverage and Close Combat for super effective coverage against Steels. However, Mega Zangoose having STAB on Close Combat makes it stronger than Return, and Tinted Lens means it doesn't need Return to hit things neutrally. This means it can run all three of Close Combat, Knock Off, and Quick Attack (still using that Normal STAB!)
and still have room for something like Swords Dance, Substitute, or Taunt.
Poison / Ghost
Terrorize (Ghost Skin)
Att: 140 (+40)
Def: 100 (+40)
SpD: 80 (+20)
In Japanese, all the -ate and -ize abilities (including Normalize)
end with スキン which Bulbapedia translates as "skin" (Electric Skin, Freeze Skin, etc.)
. That suffix isn't fully unique to those abilities (Wonder Skin also uses it, though Dry Skin and Rough Skin use はだ and Shed Skin is just "Molting"; I get the sense that it's not referring to literal skin but I don't know Japanese so I can't provide my own translation)
but even still I think it's a more elegant way to refer to that genre of ability than "-ate and/or -ize".
Zangoose's animosity toward Seviper and Poison-types in general is decently reflected mechanically through its abilities, but Seviper's reciprocal is mostly just flavor text. Mega Seviper changes this. With its new Ghost typing, it's completely immune to Zangoose's STAB, and Terrorize makes Mega Seviper's Returns and Double-Edges a cool type instead of a dumb stupid type for worthless losers. It also makes those moves completely incapable of hitting said worthless losers, but when you're being spiteful and petty you tend not to fully think things through. Also Seviper is now sort of dead, that's another thing it thought was a good idea at the time.
Mega Seviper takes a typing that was previously only seen on Gengar, and gives it a more defensive and physical spin. Its decent bulk helps it to set up with Coil or Swords Dance, as well as play defensively with Haze, Dragon Tail, or Glare. On the offensive side, it can deal massive damage with Ghost-type Return (even without any investment it's almost as strong as Specs Gengar's Shadow Ball is on the special side)
and it can use Glare to patch up its slow Speed. It can also work around its slow Speed with Terrorize-boosted priority. It doesn't get Extreme Speed or Quick Attack, but it does get Feint. It may be weaker than Quick Attack, but its +2 priority means the opponent can't outrun it with their own priority move (most notably Sucker Punch).
I really wanted to make Mega Seviper a special attacker so it could make use of Wring Out, but its movepool is just so much better suited for physical attacking, and it would also put it in even more competition with Gengar.
In terms of visual flavor, Seviper is one of the spookier-looking Pokemon, especially compared to other snake Pokemon, and its two-tone face can be expanded upon into its face physically splitting open, like its skin is mid-shed and hanging off its body like a cloak or mummy wrappings.
Rock / Psychic
Att: 125 (+30)
Def: 115 (+30)
Spe: 110 (+40)
Something I think is neat about Solrock and Lunatone is that they used their shared typing in very different ways when they first debuted. Lunatone was a special attacker, and because the physical/special split hadn't happened yet, it focused more on its Psychic typing. Meanwhile, Solrock was a physical attacker, so it focused on its Rock typing. Because of this, I think it's only fitting to treat Mega Solrock as a primarily Rock-type mega, and Mega Lunatone as a primarily Psychic-type mega when we get to it.
Like many a gen 3 Rock-type, Mega Solrock gets Rock Head without any STAB to benefit from it. However, it does get Flare Blitz, letting it smash through the Steels that would wall its STABs without suffering any recoil. It's also remarkably fast for a Rock-type, which in combination with Swords Dance can make it a very effective cleaner.