Since there isn’t going to be custome abilities anymore, here are my proposed suggestions
Ability: Toxic Chain
Toxic Chain is the signature ability of the Loyal Three and is in general a pretty good ability. Victreebel gets access to Knock Off, Bullet Seed, Sucker Punch, and some other utility attacking moves that would benefit from it. It also can activate on either attacking side which fits Mega Victreebel’s stats at least.
Still pretty bad, but stronger than just poisoning Steel and Poison types.
Ability: Electric Surge
There really isn’t a way to all that effectively salvage Mega Eelektross while making sense. This is the best ability for it that fits Eelektross.
No custom abilities is gonna devistate all the mono Electric megas since it’s really hard coming up with fitting abilities for them. It’s basically only Electric Surge, Transistor, and the Electric immunity abilities.
Ability: Dauntless Shield
Honestly Chesnaught should had this ability since the start. Not only fitting it more, but also being a better ability in general. The old custom ability was nice, but it’s only be marginally helpful. +1 Def gives Chesnaught one of the strongest Body Presses in the game without set up or Tera. +1 Def is also very useful for setting up too.
Ability: Magic Guard
Floette getting a brand new ability is unavoidable. It’s a super special Mega. If there was an exception to which Mega gets a custom ability, it should be Floette-E.
But in case there is no exception for any Mega, then Floette should get Magic Guard.
Magic Guard fitting Floette since it’s almost an evolution of Flower Veil. Flower Veil blocking all status, and Magic Guard blocking the damage from status (which is technically worse since it doesn’t block para, sleep, or freeze) and all other indirect damage.
As you can tell, this also greatly benefits Floette because of its signature move having the downside of causing indirect recoil damage.
Ability: Suction Cups or Truant or something bad.
Ok there is no ability that can make Mega Malamar good without being broken without custom abilities.
A better version of Contrary wasn't even enough to save Mega Malamar from being terrible, so Contrary itself is off the table.
Wonder Guard/Shadow Tag would be too much since it only has 2 weaknesses and unlike Shedinja has an actual HP stat too.
Huge Power might make it alright, which sounds absurd to say, but Mega Malamar has issues with actually attacking because its so slow and frail despite of its mega focusing on bulk. That isn't also mentioning how Huge Power, Wonder Guard, STag have nothing to do with Malamar.
And you know what? What if Mega Malamar being absolute garbage was GameFreak's intention. Almost every Mega is designed to be at least a "side grade". Whether those intentions become reality, like with Mega Garchomp, is ultimately more circumstance of how things play out. But GameFreak is no stranger to making intentionally bad Pokemon, moves, abilities, etc. with 0 thought of it ever actually being good. One of the most famous Pokemon is Magikarp which is supposed to be useless and learns a literally useless move (outside of Gen 7).
What if GameFreak's intention was to subvert our expectations of Megas? We expect megas to be better or at least have some use case over their regular form. So what if GameFreak intentionally made Mega Malamar bad?
This would surprisingly fit the Malamar line. The entire line being about inverting things. To evolve Inkay, you need to invert your 3DS/Switch/Iphone. They get Contrary because that ability inverts stat changes. Its signature move is about inverting the target's stat changes. Everything about the Malamar line is about reversing things. Even the stats of Mega Malamar make reference to inversions. HP is upside down SpA, Def is upside down Spe, and the 2 and 0 in Atk swap for SpD. But what if the purpose of its stats being like that because the entire intention with Mega Malamar was to also make it bad on purpose? Making Mega Malamar would invert the purpose of Megas by having a Mega that is just way worse than the original. This of course keeping in line with the 100 BST rule that only Mega Zygarde breaks (which it can because its special and gets a powerful move to compensate). Having the absolute worst stat distribution you could have for a +100 BST. It's so bad that you can use AV Malamar for nearly the same Pokemon.
By giving Mega Malamar Suction Cups (or some other useless/awful ability), you end up with a Mega that is subverts our expectations of what Megas are supposed to be and end up with a Mega that fits the themes of the line more.
Anyways if any executive at A24 or Reddit needs a creative genius, you know where to contact me.
Ability: Stamina
I feel similarly about Mega Hawlucha as I do Mega Chesnaught. Like how Mega Ches should have gotten Dauntless Shield, Mega Hawlucha should get Stamina. It practically was advertised with Stamina in mind. And if Archaludon can have Stamina, then so can Hawlucha. It'd make good use of Stamina too. Having access to Body Press, Roost, Defog, and Encore, and having pretty good bulk with a decent speed tier.
Ability: Defiant or Tough Claws
Yeah I don't have any other ideas that aren't custom abilities. Falinks appreciates Defiant at least since it prevents Intimidate from ruining its No Retreat. Would have liked my Full Mecha Body idea.
We can also have something similar to Brass Bond where it gets Tough Claws. It effectively keeps Brass Bond since it has the same boosts for pretty much all its moves. It's also better since you don't get destroyed by Rocky Helmet or Rough Skin.
Also some bonus ability changes for Megas that are currently weak.
Ability: Refrigerate
You know what. Fuck it. Just give Manectric Refrigerate.
Manectric already got screwed over by Hidden Power being taken away. It really would appreciate having Ice coverage.
I don't care if it makes sense, it really needs to happen considering we're getting 3 whole new mono-Electric Megas and Manectric needs to make it self stand out.