Well we can talk about the rumored MegaEvolutions and how they should be improved so they can be viable in higher tiers. I know a lot of the ones that where mentioned are already pretty good so Ill go with the ones who need a boost.
Ill start with my favorite starter evolution,
Feraligatr
The stats I want this pokemon to have when it Mega Evolves are as follows:
HP-
110
Atk- 105
Def- 100
SpA- 79
SpD- 83
Spe-
97
Sp Ability-
Dry Skin
The only 2 stats that in my opinion should be boosted is HP and Speed. Both of these stats where boosted by 25%, I believe that it isn't too low to not make a difference and is not too much to be used without any drawback. The reason I picked speed to be boosted is because the main reason Feraligatr isn't a superb pokemon is of its low speed, it just doesn't meet the standards to be considered a viable sweeper and also almost of his pokedex entries describe him of being very fast so you would expect him to be at least faster than Venasaur. I choose HP over Attack because even at 97 theres still a lot of counters to Feraligatr that outspeed him, without that HP boost people would most likely go for a Dragon Dance set most of the time but with a bulkier Feraligatr you could go for both a DD set and a Swords Dance set since it can survive a lot more hits.
Now to the special ability. I know what you're thinking a water pokemon with dry skin seem overpowered, but MegaPokemon are suppose to be the strongest characters on your team no? Its a one time use in battle and we still don't know what are the requirements to do so. Anyway Feraligatr is already a good pokemon in the rain, so this ability will only make him even stronger in the rain. Also alligators have dry skin so it makes sense.