I'll start with the Regis:
Regirock:
>gets new ability: Rock Repair which heals 1/16th HP in a Sandstorm + HP is restored if hit by a rock move similarly to volt absorb or water absorb
Regice:
>gets Snow Warning as HA instead of Ice body
Registeel:
>gets new ability: Auto-repair System which heals 1/16th HP per turn regardless of weather, essentially a free leftovers
Regigigas:
>Slow start only lasts for 2 turns instead of 5
oh yeah and they all get recover
>>31495041
I was about to say, they really need a recovery move considering how defensive they are.
All of the Regis save Gigas would really benefit from the ability Regenerator as well.
I do like the custom abilities you made though. Considering how much item-ability overlap there is I've been waiting for a Leftovers ability with all the ability-creep that keeps happening each gen.
Mega Ditto
+20 to all stats but HP
Ability: Superior
Acts as Imposter but all base stats are copied at +1. Extremely useful now that speed is taken into account after mega evolving. HP is still 48 though.
Also, some mons that could benefit from abilities:
>Electrivire
Sheer Force. Given the fact that its schtick is a beefy physical electric type, I think it fits. Could also go mixed or even full special. Just look at Nidoking. Would at least be UU, balanced by its meh defense and middling speed tier.
>Articuno, Moltres, Zapdos.
Weather setting abilities. All fit very thematically. Zapdos can't really use Heat Wave reliably now though despite the nice Thunder boost. Still scary, may likely go to ubers due to pairing with Kingdra plus other swimmers.
How about making Pokemon AG?
>Regigigas gets Recover and Skill Swap
>Slaking gets Skill Swap
>>31495016
Regice needs a lot more than an ability change to become OU. It'll need some ice equivalent of shell smash or something.
>>31495469
Ditto has a simpler fix, make his powders work after he has transformed. Basically a free scarf or defense assault vest
>he doesn't use AV regice for a special wall
>+20 Speed
>+10 Sp. Atk
>+10 Sp. Def
>SolarBeam and fire attack
>Return Gen5 Meta and Infinite weather
Take 3 points off of Sp. Def and put it in speed. Oh and he now learns Sludge Wave