I doubt there will be new Mega Evolutions at this point, but I'm definitely up for new boosts for past Pokémon.
My personal choices.
1. Flygon
The most ideal, since it's the only Dragon-type in Hoenn that still lacks one to this day. I would imagine it's wings taking on the form of a sun, since Mega Salamence does resemble a crescent moon.
2. Rhyperior
It'll be a chance to both improve it's design and competitive viability.
Ideally, it should learn Slack Off or Shore Up to survive in battle much longer to really make use of it's massive HP and Defense. I envision it being Ground/Steel thanks to improved protectors and overall armor.
3. Vikavolt
I'm rather disappointed at how slow it turned out to be. It's one of the many Pokémon that's cursed with bad speed and awkward bulk, like most Alolan Pokémon. However, a Mega Evolution with Speed Boost would make things even better for this badass Bug/Electric beetle, even if it gets banned in nearly all main tiers.
4. Dewgong
It now has a strong physical movepool with moves like Drill Run and Megahorn In it's offensive arsenal. It just needs a great physical Ice-type move like Icicle Crash, since Avalanche is a Gen IV exclusive move. I would imagine it having higher physical Attack and Speed. It's horn would be strong enough to shatter icebergs.
5. Escavalier and Accelgor
Both are counterparts to each other. Given how we're set for a medieval setting in Galar, it would be a fantastic opportunity for Escavalier to receive a Mega Evolution. It could get Stance Change, which would give it phenomenal Speed and physical Attack on one stance, and near impenetrable defenses on the other.
A Mega Accelgor could be Bug/Water, since it can learn Water Shuriken. It might learn a Water-type ball move similar to Energy Ball. I also contemplated Bug/Dark for the thief motif, but it's pre-evolution; Shelmet, is normally found in watery fields in marshlands.
6. Gogoat
Mainly to have a powerful force to help defend any sacred hill in Galar. It would have an even stat distribution, since it's almost relatively balanced. I would like it to learn Earth Power, since it naturally has a decent Sp. Atk stat.