Seeing as Pokemon Champions might mean the introduction of megas to this format, I've been spending some time recently speculating on what mega evolution could accomplish in Free-For-All, and I'd like to share my thoughts:
Obvious bans:
Potential banworthy mons:

Very powerful physical attacker with Dragon Dance and Roost. Destroys everything that isn't Corviknight.

Bulky Darkrai with 165 base Special Attack and Nasty Plot. Psyshock for Blissey, Sludge Bomb for Fairies. It breaks everything but it doesn't have reliable recovery and is slow, so potentially manageable.

Destroys nearly every Pokemon besides Blissey, although it does have IronPress so it can beat Blissey that way. Has a lot of potential on sun teams. Again, this doesn't have reliable recovery, so it could be manageable.

Parental Bond-boosted Power-Up Punch sounds crazy, but it's only off a 125 base Attack stat, and this format is dominated by Unawares. I can see Seismic Toss being really good on this. It also gets Wish to heal itself and potentially others.

Very strong Nasty Plot/Calm Mind sweeper with STAB Close Combat for Blissey and Steel moves for Clefable. Doesn't get any reliable recovery so will pair well wish a Wish user or pivot.

Its STABs threaten the two best Pokemon in this format. No reliable recovery and 80/80/80 defensive stats, so will function as a wallbreaker who needs support. Then again, might not actually be that good since regular Blaziken is never used here.
Metagame staples:

Swords Dance or mixed sweeper with no reliable recovery. Really struggles with Skeledirge or Dondozo if that is freed when Champions releases, and mixed sets might struggle into Tera Ghost Blissey and Idef Corviknight.

Rapid Spinner with reliable recovery. Bulk Up sets are going to be powerful, this will also be really strong on rain teams. I'm really excited to use this Pokemon.

Basically a sidegrade to Kyurem-Black with more bulk and slightly more Attack but less speed. Uses a mega slot, but this also has Swords Dance, Icicle Crash, and Glaive Rush which might make it worth using over Kyurem-Black. Not having Teravolt will be pretty disadvantageous, but it gets High Horsepower for Skeledirge and if it keeps Thermal Exchange when it mega evolves then it won't have to worry about burn, which is nice.

Good attack and speed with access to Swords Dance, but it's really frail so it might struggle. We'll have to see, it might be good with defensive investment.

Dragon Dance and Mold Breaker with STABs that hit the two best Unaware users in the format for Super-Effective damage is really cool. It's weak to rock before it mega evolves and doesn't have reliable recovery, so it will need some support to lean on.

Psyshock combined with special coverage and Recover feels really powerful, like it's a more offensive Galarian Slowking. These also both get Wish for support, and Latios gets Psychic Noise.

It's really bulky and has Swords Dance and Roost, so this could be a really powerful metagame threat. Its coverage options are a little lacklustre but it has high Attack and Speed, could potentially flinch down bulky mons with Iron Head.

This has a stronger Volt Switch than Regieleki (before Specs), so it might be good. Unfortunately it doesn't have an Ice move. It can still Knock Off into most Grounds and make progress, but this gets walled by Gliscor and has no reliable recovery.

This might be really good, seeing as losing speed doing matter much in this format and the extra bulk and Attack will help this Pokemon out a lot. I really want to try making a Sand team once we get access to this.

Dragon Dance and Roost is really nice. It's more frail than Gouging Fire and can't hold Leftovers or Boots, but has more Attack and Speed, and a better ability when Sun isn't up.

This has really good bulk plus Swords Dance and Synthesis, so this could potentially be a really scary sweeper. It also has Iron Defense and Body Press coming off of a 172 base defense stat.

This has Nasty Plot, could be a decent rain abuser and it has good offensive typing but it has no reliable recovery.

Decent special/mixed attacker with reliable recovery. Not being able to Terastalize this will suck, though, and regular Dragonite has a higher Attack stat and would probably be better as a Dragon Dancer.

Could be a decent Pokemon on rain, but regular Greninja is almost never used here so it might suck. I like the idea of getting a KO to activate Battle Bond before mega evolving, but it sounds inconsistent.

This has 175 base Special Attack, so once Blissey and Clodsire are out of the way this thing can go insane. Its only recovery is Pain Split, which might limit this in some situations, since it also can't hold Leftovers or Boots. You could also Psych Up on Calm Mind users or just Calm Mind yourself, run Substitute+Curse+Pain Split, it even gets Taunt so this could stop other Pokemon from recovering HP in certain positions.

Really powerful slow bulky attacker, you could go for Bulk Up sweeps with Alomomola support, this could potentially be really strong.
Other random Pokemon that I want to talk about:

This could be a decent Shell Smash user but its stats are kinda bad and this has no reliable recovery.

Probably will never be used over regular Clefable.

No reliable recovery and gets hard walled by Corviknight. Some Swords Dance potential, its Attack is really high, it just needs a teammate to deal with Corviknight.

Pure Dragon type and has less Defense than base Garchomp. Nasty Plot just gets walled by Unawares, and Latios and Dragonite have harder hitting Draco Meteors.

Probably trash if it doesn't get reliable recovery.

Not worth using over regular Slowbro.

Hippowdon has better typing and reliable recovery.

Potential Dragon Dance Roost user

Loses half its HP to Stealth Rock and can't hold a Heat Rock, but could potentially work as a sun setter that can take advantage of its own sun.

Walled by Unawares.

Potential Magic Bounce user

Potential sun abuser

This has reliable recovery and all the good special Fairy moves, and high Special Attack and Special Defense, but has very weak coverage and very little utility, so probably outclassed by Clefable.

Potential threat with Sharpness.
Overall, I'm really excited to play Free-For-All with mega evolutions and create new powerful teams in the Pokemon Champions metagame. I'd be excited to see if anyone else has thoughts about these megas. I'm particularly excited to use Latios, Starmie, and Skarmory.