MAKE (BASE-FORM) NECROZMA WHITE Seriously, wouldn't that be so much cooler than the random dark blue?!
Also, while we're at it, make shiny Cosmog red like its evolutions.
On a similar note, I'd also change Arceus - I feel that the neutral color scheme aside from the ring, etc. was designed so it would go with any colors when you changed the type, and the yellow just completely throws that off and also reminds me of scrambled eggs. I'd just switch the white parts of the body and the grey parts.
Appletun's shiny is just not at all striking, Flapple's is a bit better but still sorta just blends into a blur of green, so I'd keep the green apple thing but make their bodies red. Maybe Applin as well. (I'm realizing as I write this post just how much I like inverted shinies.)
I'd have Blissey, Typhlosion and Haxorus keep the same color scheme as Chansey, Cyndaquil/Quilava and Fraxure (with the blue accents) respectively.
I don't hate Houndour and Houndoom's shiny forms, but I'd prefer dark red.
This is a common one, but: autumn-colored Leafeon instead of the barely tinged form we have now.