If anything, I feel some Megas suffer the Gen 4 "over decorated" evo problem
That said, if we're to turn them into evos for design, personally...
Audino: tweak ear shape slightly
Sableye: perfect, maybe change purple
Beedrill and Ampharos: change yellow to be oranger
Altaria: cover eyes with fluff
Glalie: iffy on design, but it's distinct enough already
Aggron: I can't think of much
Salamence: it's a stupid design, but maybe make it greener. Or ref the RSE color
Camerupt: fine already
Blaziken: fine already
Sceptile: fine already
Swampert: good already
Gardevoir: have it more like a knight with flowing elements
Gallade: ^shrugs^
Aerodactyl: recolor it
Slowbro: This is horrible lore wise. Keep it
Lopunny: no changes
Tyranitar: meh. I dunno, but meh
Metagross: No changes
Sharpedo: Recolor spikes
Pinsir: recolor body to be darker, metallic
Heracross: fix the face, change color
Scizor: make it blue, rgb Evo line!
Alakazam: beyond perfect
Banette: perfect
Abomasnow: It's design is garbage, please redesign
Steelix: maybe make body black. Onix's dex entry notes that really old ones are smooth and black
Houndoom: recolor bones
Medicham: Gold instead of pink
Manectric: I feel a recolor is needed, unsure
Pidgeot: Perfect
Gyarados: Perfect
Mawhile: Change color (slightly or not), the second head and body shape is nice
Absol: maybe have it grey, to be a fallen angel
Gengar: change eye color, shiny differentiates nicely already
Kangaskhan: This is a weird case where I'd say make it so the pink baby for reg is first form, second is the mega of the kid, then final Kangaskhan
Venusaur: the design is trash cuz butt flower cutout
Blastoise: extra notches or maybe fluff, a bit plain otherwise
Charizard X: make the black shiny, color closer to orange, but retain red eyes
Charizard Y: change belly color
Mewtwo, Rayquaza, Kyogre, Groudon, Latis: No, you're legendaries, stahp