Predicting

and

stats and ability.

There is like already 4 Megas in ZA that have a “Bell Curve” stat distribution, where the 100 points in their stats resemble a bell curve. You got

and

which all have stat buffs that resemble bell curves.
So a possibility for Mega Chimecho would be
HP: 75
Atk: 60 (+10)
Def: 100 (+20)
SpA: 135 (+40)
SpD: 120 (+30)
Spe: 65
Ideally, Chimecho would get the Mega Beedrill treatment and cuts a lot from its weaker offense to put into its other stats. Though +70 in SpA and Spe would be very much unrealistic, so it could be also.
HP: 75
Atk: 10 (-40)
Def: 140 (+60)
SpA: 145 (+50)
SpD: 150 (+60)
Spe: 35 (-30)
This stat distribution would work very well for Chimecho’s movepool and the new Steel typing it has. You are extremely bulky with your 75/140/150 bulk, multiple resistances, Recover, and can’t be Toxic’d so its very easy to set up multiple calm mind or cosmic powers for Stored Power.
Additionally Chimecho learns Trick Room so it can take advantage of its lower speed rather than being at an awkward 65 Speed that’s too fast for Trick Room but slow compared to everything else.
But we’re also talking about GameFreak here, and Chimecho could very well get Mega Pyroar’s stat distribution of +20 everywhere, which is worse for Chimecho because at least Mega Pyroar is decently fast. Mega Chimecho does get 6 smaller Chimecho and is basically a big ring, so having an even distribution of stats would not be out of the realm of possibilities.
As for abilities, most obvious ability would be Levitate. The entire line gets Levitate and you know its floating. Plus that added Steel type would mean you’re only weak to Fire, Ghost, and Dark. Chimecho is kind of like Bronzong too, being a psychic type bell.
It could also get Sound based abilities. A Steel type Liquid Voice would be nice for Psychic Noise to let you hit Dark types, and also because Chimecho currently doesn’t have Steel STAB at all.
Personally Levitate would be the better option. An immunity to Ground type attacks is great for a Steel type, and an immunity to Spikes while resisting Stealth Rock is great for a Mega Pokemon since Megas can’t hold HDBs. Speaking of.

Mega Baxcalibur is gonna need a lot for it to work, despite its Uber status. That is because Baxcalibur’s brokeness is hard carried by items and outside help. If Mega Bax doesn’t get insane stat buffs, why use it when you could use HDBs, Leftovers, or Loaded Dice? Bax was also very heavily carried by Alolan Ninetales. Aurora Veil+Snow giving Bax x3 Def and x2 SpD, while also getting chip healing from Ice Body. Oh and its signature ability prevents burns too and boosts its Atk of you hit it with a Fire move, sooo…
How Baxcalibur will distribute stats, I have now clue. It has a massive Ice sword so it could be something like.
HP: 115
Atk: 185 (+40)
Def: 112 (+20)
SpA: 75
SpD: 116 (+30)
Spe: 97 (+10)
This feels like a stat distribution you could use for Baxcalibur that’s be worth running your Mega Slot and Item Slot with.
However the biggest point of interest is Bax’s ability. Depending on what it ends up being, you might as well use its base form for Thermal Exchange.
Which it should get since Thermal Exchange is Bax’s identity for the most part. It’s a great ability for Baxcalibur as a physical attacker as you boost its attack and make it immune to burns.
If it gets Sharpness, just throw in the towel. Currently it only gets Aerial Ace to abuse Sharpness on, which could change, but not worth giving up +1 Atk from being hit by fire moves and burn immunity. Sword of Ruin and Intrepid Sword would be unrealistic since both are signature abilities of legendaries with only other legendaries sharing similar abilities (at least Drought/Drizzle are similar to Sand Stream).
We could also get a Mega Garchomp 2.0, which itself is Kyurem-Black 2.0.
Instead of boosting speed, it gets slower and a bunch of its BST is wasted on SpA, then gets Snow Force that boost just Ice moves by 30% in Snow. Or goes the Mega Dragonite route and becomes a Special Attacker.
I’m not giving those alternate stat distributions because looking at them would be as disturbing as that Funky Town video. You know, you know.