Im glad customs are going tbh, I always felt that pretty much every new mega could work with an already existing ability.
The only two who i felt deserve custom abilities were

-mega and

-mega. Although Mega Victreebel could work with something like Supersweet Syrup or Toxic Debris, Mega Malamar was absolutely hopeless, with the only abilities that could save it that I can think of are Prankster (the most likely candidate), Wonder Guard (absolutely out of the question), and Huge Power (

anyone?).
Given the incoming metagame changes with all the new pokemon in the DLC, and the ability changes, i decided to do some predictions.

Mega Eelektross: Ion Battery > Earth Eater or Electric Surge. Eelektross's whole identity is having no weaknesses, so giving it Earth Eater seems pretty simple change. However, Eelektross is notably lacking in power compared to other Megas, so I can also see Electric Surge being possible. but i doubt it.

Mega Malamar: Contrarian > Contrary or Prankster. Prankster Topsy Turvey would actually be amazing against a lot of the setup sweepers such as

Mega Skarmory, and it also gets Destiny Bond so we can use Prankster Destiny Bond and have it actually do something other then nuke 1 or 2 mons

Mega Falinks: Brass Bond > Defiant, Mirror Armor or Simple. The Speed Boost Mega Falinks propoganda has left my brain and i have finally reasoned with 2 potential and less broken abilities. The former 2 would block Intimidate, which is pretty much neccessary for Mega Falinks to be viable, but Simple would allow it to get +2 off No Retreat, which would be extremely powerful. The only issue is that Mega Falinks also gets Swords Dance so that might end up being better with Simple, and it also means Intimidate sends it to -2, so it might make it very matchup fishy.

Mega Chesnaught: Protective Thorns > Dauntless Shield. While Bulletproof would be technically more useful for it, Dauntless Shield Mega Chesnaught would allow Chesnaught to play in its base form to dodge moves like Shadow Ball, and then mega evolve later to endgame with a Dauntless Shield boost.

Mega Hawlucha: Luchador's Pride > Stamina. I dont know why we gave it a speed boosting version of Moxie, when Stamina not only makes it a strong Body Press user, but also matches its dex entry. Although i do wish we could see a world where we give Mega Hawlucha Tinted Lens just to see if it would be viable with Flying Press...

Mega Floette: Ange > Fairy Aura. Ange is bullshit, and because Floette lacks any sort of coverage other then Grass, it might aswell boost its Fairy moves substantially.
I cant really think of any relavent calcs, so i'll throw this one here just to show how impactful it would be
252+ SpA Floette-Mega Light of Ruin vs. 252 HP / 0 SpD Tyranitar-Mega in Sand: 326-386 (80.6 - 95.5%) -- guaranteed 2HKO
252+ SpA Fairy Aura Floette-Mega Light of Ruin vs. 252 HP / 0 SpD Tyranitar-Mega in Sand: 434-512 (107.4 - 126.7%) -- guaranteed OHKO
It also means Zygarde actually has an ability in exactly 1 matchup instead of 0, so thats nice.

Mega Mega
Meganium Flower Veil > Triage. Why on earth did we ever think Triage would be bad on a bulky Mega??? I get it was early days, but people have been asking for Triage Meganium since forever, so it should get Triage.

Mega Excadrill: Mold Breaker > Sand Force or Rocky Payload. Mega Excadrill actually needs a relavent ability for it to be viable, despite being Excadrill. Sand Rush would probably make it too overwhelming, so Sand Force actually makes sense here. Although I think Rocky Payload would also make sense even if it can only be used with Rock Slide.

Mega Victreebel: Corrosion (+hit Steels) > Toxic Debris or Supersweet Syrup. Chlorophyll doesnt make sense because we have no sun setters. Supersweet Syrupt would allow it to use Power Whip or Leaf Storm without fear of missing, and Toxic Debris would allow it to set Toxic Spikes which, given its mixed attacking stats, would probably give it a niche.
okay im tired goodbye smogon forums