Young Battle Trainer
I'm thinking that there won't be any new Megas. It looks like its a Gen 6 gimmick.
What about Mega Torkoal.
Concept: When it mega evolves its shell turns into something similar to a "Steam Engine". With its its red spots morphed into air vents. Its will be a more defensive Pokemon.
Ability: Extreme Heat - The ability evapourates/melts water/rock type attacks making the Pokemon immune to them.
BTS - 70/100/150/100/150/20
That's a +120 to BST, normally mega evolution only gives +100. And remember that base 100 in an attacking stat isn't bad, it's capable of inflicting good damage especially with Torkoal's sun boost (which it'd set up prior to mega evolution). Tapu Koko only has base 95 Sp.Atk and it hits hard with Thunderbolt thanks to Electric Surge, you're looking at those kinds of damage ranges here, only on a very bulky pokémon whose only weakness is ground attacks and with access to a base 150 power move in Eruption.
Sadly, I don't think that they are making any more megas.Has anybody mentioned that Rapidash would be an amazing candidate for a Mega? I for one imagine it being a Fire/Fairy Type? I mean it'd make sense being a unicorn and all. boost her speed up to 120 and make her a mixed attacker? any takers?
It appears that you have forgotten about the signature Z-Moves.Asking for them to bring back Mega's after they already ditched the concept is just weird in my eyes. Z-moves allow for far more diversity in strategies and you just need 18 items and not keep on adding a new item for every new Mega Evolution.
Just something random and interesting I thought but you know what I just realized about the recent leaks reporting about the 15 mega evolutions? Three of them are Mega Evolutions based on the Pokemon that the Elite Four have. Jynx, Dragonite and Machamp. Gengar already has one. Take the leak with a grain of salt folks. Honestly I wouldn't be surprised if it turned out there were no mega evolutions, as Let's Go is a game about catching over battling.