Personal wish list of new megas (mostly ones that could even maybe possibly happen because Sun/Moon motifs):
Clefable (but balanced pls)
Starmie
Gen 2 Starters (cuz another Gen 2 remake isn't coming anytime soon)
Noctowl
Sunflora
Umbreon / Espeon
Xatu
Bellossom
Flygon (for fuck's sake)
Cacturne
Lunatone / Solrock
Grumpig
Tropius
Cherrim
Mismagius
Weavile
Musharna
Sigilyph
CHANDELURE PLEASE
Volcarona
Gen 6 Starters (if not Gen 2 ones, cuz it seems like a good time for these to get megas)
Heliolisk
Noivern (just cuz)
Design wise I find Reuniclus to be a prime mega candidate it's so easy to imagine them giving it more skeletal limbs. And Flygon has to happen, my dragon team craves it.