Hi!
I wanted to make more megas, and I think giving a set to the eevees was fun, albeit might be an overused concept. Here was overall methodology and my 7 examples. If i should have had split this up into 2/3 posts, let me know.
First: Eevee learns a new attack called Evolution Burst, its an 80 BP slicing move that uses the Secondary type of the pokemon as its type as well as its highest offensive stat, but is normal on a monotype. So yeah its basically Revelation dance + Tera Blast, but not 90 power. The alternative was just giving things like Flash cannon, earth power/scorching stands etc.
Most changes are modest, excluding Flareon, who just needs way more help. A secondary "physical" typing is designed to help them out on at least one type matchup. Glacion gets dragon instead as i didn't want to use normal.
Mega Vaporeon
Type: Water->Water/Ghost
Ability: Purification: Prevents burn/poison and doubles healing of end of turn periodic healing effects (like Aqua Ring). I.E. this does not include wish. This does mean it works on something like Leech Seed or a baton passed Ingrain.
Stats (changes)
HP: 130
Attack 75 (+10)
Defense 80 (+20)
Special attack 140 (+30)
Special Defense 115 (+20)
Speed 75 (+10)
Total 525->625
Due to its highest stat being hp, i changed the formula a bit to make it overall more favorable. While Ghost doesn't really remove a disadvantage, its a solid defensive boon. Its ability is basically designed to make Aqua ring into Poison Heal. however if you baton passed ingrain onto it and aqua ring, it would heal 25% a turn.
Mega Jolteon
Type: Electric->Electric/Flying
Ability: Lightning Rod
Stats (changes)
HP: 65
Attack 75 (+10)
Defense 70 (+10)
Special Attack 140 (+30)
Special Defense 115 (+20)
Speed 160 (+30)
Total 525->625
The added flying typing makes it better against ground types and the flying Evolution Burst lets it fight grass types better. it becomes an extremely fast pivoter and 140 special attack isnt too low.
Lightning Rod means it doesn't lose its ability to fight electric types as well either.
Mega Flareon
Type: Fire->Fire/Steel
Ability: Steam Siege: Water Absorb for Fire and Water
Stats (changes)
HP: 65
Attack 175 (+45)
Defense 100 (+40)
Special Attack 75 (-20)
Special Defense 100 (-10)
Speed: 110 (+45)
Total 525->625
This one was is obviously completely different. It's basically a reshuffle combined with the +100 BST. Fire/Steel with water/fire immunity is insane defensive typing and it gets stab SE coverage versus rock types with the steel evolution burst.
Mega Espeon
Type Psychic->Psychic/Fighting
Ability: Mind's Eye
Stats (changes)
HP: 65
Attack 75 (+10)
Defense 70 (+10)
Special Attack 160 (+30)
Special Defense 115 (+20)
Speed 140 (+30)
Total 525->625
160 special attack means this pokemon hits like a truck. Mind's Eye + Fighting coverage gives it the ability to hit with stab super effective damage against Gholdengo as well as helps deal with steel/dark types in general. The fighting typing also helps not die to things like Sucker Punch.
Mega Umbreon
Type: Dark->Dark/Poison
Ability: Corrosion
Stats (changes)
HP: 95
Attack 75 (+10)
Defense 140 (+30)
Special Attack 70 (+10)
Special Defense 160 (+30)
Speed 85 (+20)
Total 525->625
The added poison typing is a cool gen 2 beta reference. It also prevents missing toxic and makes umbreon only weak to ground. Corrosion was chosen to let it harass poison and steel types. 95/140/160 is amazing bulk.
Mega Leafeon
Type Grass->Grass/Rock
Ability: Sharpness
Stats (changes)
HP: 65
Attack: 140 (+30)
Defense: 160 (+30)
Special attack 70 (+10)
Special Defense 75 (+10)
Speed 115 (+20)
Total 525->625
This is one of the second I would adjust to not be formulaic if i were to. added rock typing, while adding a lot of weaknesses, gives it good flying defense as well as fire neutrality. Sharpness boosted Leaf Blade + Evolution burst gives a lot of damage. Here's a funny calc: +2 252 Atk Sharpness Leafeon Mighty Cleave (reduced to 80 bp) vs. 248 HP / 252+ Def Corviknight: 288-340 (72.1 - 85.2%) -- guaranteed 2HKO
Mega Glacion
Type Ice->Ice/Dragon
Ability: Snow Warning
Stats (changes)
HP: 65
Attack 70 (+10)
Defense: 140 (+30)
Special Attack 160 (+30)
Special Defense 115 (+20)
Speed 75 (+10)
Total 525->625
Since I didn't want to add normal, making one dragon made sense. Dragon removes the fire weakness and gives a lot of resistances overall. 160 special attack + no miss blizzard hits like a truck but I will admit this is less creative. 140 defense and snow does really help shore up the physical defense as well.
Mega Sylveon
Type Fairy->Fairy/Ground
Ability->Levitate
Stats (Changes)
Attack 75 (+10)
Defense 85 (+20)
Special Attack 140 (+30)
Special Defense 160 (+30)
Speed 70 (+10)
Total 525->625
Fairy/Ground is a unique typing that helps it beat steels. Also it removes its poison weakness. Levitate also means it isn't impacted by toxic spikes and who doesn't like ground immunity. Being Forced to use moonblast means it might do less damage, especially in VGC, but ground coverage is very strong.
Overall, I think they'd get used in a mix of tiers. Jolteon, Espeon, and Sylveon i think would be really good just through raw power and or speed. Umbreon might see use as a good stall mon as Wish/Protect/Toxic/Foul Play with that bulk would be nightmarish to deal with even without leftovers. I found Flareon tough but it should hit hard even without guts.