Art by: Jeremon (on X), Onduregion (on IG) and CardnIH (on X)

My proposal for how these three should look like stats and ability wise. I made thinking mainy of singles and gen 9 national dex (even though they'll exist on gen 10).
I design these Megas' competitive proposals and many more for this video, so check it out if you are interested :D
[11 new megas and how to design them (competitive)](https://www.youtube.com/watch?v=sUQNHgWa9xU)

Delphox: 
Wanted to keep her in her role of fast special attacker, but with more utility and/or survivability, which is why for an ability she would have Magic Guard/Magic Bounce/Magician’s trick (1 or 2).
Magician's trick: Each time this pokemon hits an enemy, it knocks off their item // When this pokemon hits an enemy, it makes a copy of their item and gains the effect. Once per switch in and the effect is removed when switching out.
For the custom ability, I wanted to make a buff/new version of her base ability Magician, but it was definitely a little hard to design considering as a Mega, she would already have an item. 

Chesnaught:
Wanted to be able to keep his defensive role but also improve him as a bulky slow attacker, but not so much as a wallbreaker, but rather as a slow bulky win condition, with a greater focus on healing itself. Which is why I gave him the following custom ability (two versions):  
Leeching spikes: Life steal moves have 75% increased life steal // Life steal moves deal x0.33 damage.
I kept movepools mostly the same, with the exception of Chesnaught who I would give access to horn leech (which I thought weird it didn’t have already). 
As an alternative, I gave him Moxie, to help with this idea of a slow snowball mega, but I don’t know how well it would work on a slow attacker.

Greninja:
Went with something similar to Greninja-Ash but less potent and less frail, with a greater focus on its pivoting and mixed attacking abilities. With a huge speed and a varied movepool, mega greninja would be versatile and a pivot that is constantly coming in to revenge kill and then pivot out. Especially with a new custom ability that would buff his stronger base damage moves:
Ninja's Precision: This pokemon’s type changes to the type of the move it is using. Once per switch in. This pokemon moves can’t miss.
So protean + perfect accuracy hahshash. It’d be particularly good for Blizzard, Hydro Pump and Gunk shot.

Mega Delphox, Chesnaught and Greninja Competitive proposal