View attachment 841304
Ok so going through the mons one by one:
Raichu X feels good as an uncontested terrain setter in this format, though there isn't much that benefits from Electric Terrain. The most viable Rising Voltage users in the format are Raichu X itself, which it can use to bypass recoil that would be there from a Volt Tackle set, or Jolteon. It is a nice secondary mega against rain though imo, being able to redirect Electro shot and as a bonus it counters both Raichu forms without Mega Evolution. Being able to threaten Zard and to an extent Floette (32 Atk Raichu Volt Tackle vs. 2 HP / 0 Def Mega Floette in Electric Terrain: 153-181 (101.3 - 119.8%) -- guaranteed OHKO) is nice.
Raichu Y is cool and all, with Zap Cannon for Yard and Focus Blast for Gambit. 160 base Spa is really impressive too. Thankfully the only real competition for special electric is Rotom-W, so maybe Raichu Y will see niche play among Raichu teams, since I'm not sure Raichu will be a primary Mega in MB.
Vileplume isn't much to talk about, it could do something with Acid Spray + Chlorophyll next to Yard but that's all I see.
Qwilfish is never making it in a format with Toxapex.
Sceptile getting Earth Power is cool and all, and the speed is nice, but lightning rod isn't an amazing ability for a mega and its coverage is bad into the established metagame.
Blaziken I actually have serious hopes for. Sash Coaching or baton pass sets can help boost the team, and Swords Dance Mega Blaziken provides an alternative to Xard on certain teams, single turn of Swords Dance + Protect is two Dragon Dance worth.
Swampert is going to be really nice for rain teams, though does face an issue that rain will end up tripling waters with Basc Peli Pert. Earthquake next to Pelipper is really nice to go for off the bat considering Incineroar is hard to bring into rain. Being immune to Electric is quite nice too if Raichu gains popularity. Ice Punch is probably the play of third move to stave off Garchomp, though I can see options like secondary mega Mega Swampert using Wide Guard. (Just heard it got wave crash too which is a crazy buff)
Mega Mawile has a base attack of effectively 261. It honestly seems like a nice mon for Trick Room teams, Rock Slide can threaten Yard pretty heavily if you have the speed advantage.
Mega Metagross is a really good addition into the metagame, it outspeeds Floette and Garchomp with bulk to spare, coverage wise its got huge defense for body press, Ice Punch for Garchomp, Rock Slide for Charizard, Zen Headbutt for Sneasler/Toxapex/Venusaur/Annihilape, etc. We might see a lot of Jolly sets without Bullet Punch and then Bulky Bullet Punch adamant sets.
Staraptor is honestly not too bad, its an intimidator that can boost its defenses, so can take full advantage of Sinistcha. Being weak to Floette is kinda ass though so we'll see how it does. Base 110 for tailwind is really neat however, especially with intimidate to mitigate the aero weakness. Could see play as an aero alternative.
I can't see Musharna doing anything this format.
Scolipede mega unfortunately doesn't get enough to work. Iron Defense speed boost sets are probably the best but there's a million and one set up sweepers around and many of them don't get Heat waved away by Yard.
Scrafty with two intims in one is nice but I feel like you mega so rarely and you're so weak to sylv/Floette that you'd rather just use Incineroar.
Eelektross getting Beast Boost is p cool but its movepool is what holds it back, maybe it got something new. Speed stat is also a bit awful.
Pyroar is just far worse yard, Malamar did not get saved, and Barbaracle got worse typing and only 600 bst to show for it.
Dragalge feels like a Mega Toxapex, good defences, maybe an Acid Spray set could be nice as a very slow position heavy team.
Grimmsnarl losing twave is a bit meh, but it still has plenty of tools. Fake Out, Parting Shot, Sucker Punch, and Scary Face come to mind. Grimmsnarl could either go Roseli or Light Clay, depending on the pace of the team.
Mega Falinks after an omniboost is a pretty funny cheese team idea, might not be the most serious but its funny.
Overqwil's typing is amazing, Gunk Shot and Throat Chop are really good into Floette and Incin, and its got a decent support pool.
Houndstone I can definitely see next to basc on sand for a really brutal last respects team.
Annihilape and Gholdengo suffer a lot from losing terastallization, though the current metagame is very conducive to set up and they can pair with things like Mega Delphox as a secondary set up sweeper. The Rage Fist nerf means Ape is more or less stuck in one place, probably meaning the bulk up will remain THE ape set. Gholdengo's MIR nerf hits hard for consistency but its still a threat, I think on rain this thing could actually do well since Swampert actually does well into a lot of Gholdengo counters. Choice Scarf Power Gem Gholdengo could also be really good on offense teams.