Introduction & Goals
Behold for it is I, the master of illusions, the wearer of fishbowls, the guy who isn't gonna keep this act up while writing this super long Forums post. If my name and profile didn't give it away yet, I love Mysterio. He is my favorite Marvel character. Since I love the character of Mysterio I want him to be a meta relevant champion in MCOC so I (and others) get to play with him more! Consider this post mainly a passion project from a fan of the character.

For this I will have a quick look into the current kit, the issues with it and then propose some changes for Mysterio. For this I have some important goals in mind to keep this concept realistic:
1) Make sure the new kit is befitting of the character his abilities and lore in the comics.
2) Make the champion meta relevant, but not make an overpowered champion.
3) Respect Kabams design philosophy and limitations. As you can see
here, Kabam clearly intended for this guy to have some defender strength. I also want to ensure Mysterio respects the class wheel, so he should be countered by cosmic champions and be feared by Mutants.
Current Kit & It's IssuesFor anyone who wants to read about Mysterio his current kit, you can use the previous link to check out the offical MCOC website where it's all explained in detail. Personally I will be going over the key abilities quickly and then pointing out the biggest issues in his kit holding him back in the current meta.
His current kit grants him the following:
1) As long as he is not armor broken or knocked down by a #Spider-verse Hero he has immunity to poison and his ability accuracy cannot be reduced.
2) Sp1 applies a damage reflection effect that heals Mysterio for all damage taken while it was active.
3) Sp2 applies a powerdrain on the opponent and gives Mysterio some time where some attacks may miss, this goes up to 100% chance to miss if he is charging a heavy.
4) Sp3 gives Mysterio a big damage boost for a set amount of time.
5) Ending a combo on a 4th light attack applies a passive heal block.
6) Using a heavy applies a poison, against #Spider-verse Heroes this disables their evade while the poison is on.
In theory you'd think Mysterio would be super good, heal block, power drain, miss, immune to AAR, poison immunity, poison damage, evade counters on specific fights, big damage boost. However there are a few glaring issues holding Mysterio back:
1) Nearly every single ability relies you to have Chemical Gas charges. All the special effects listed above as well as the passive heal block have super short durations to the point that you cannot really say they do anything for you if you are not at 3 Chemical Gas charges.
2) Using Chemical Gas charges means you have to build them up again before accessing another piece of utility. Need a big power drain? That means you won't get big damage or poisons!
3) Mysterio has no ways to gain power during the fight, so if you wanna access your meaningful damage you will need to build up to 3 bars of power, then build up to 3 Chemical Gas charges again, then heavy. Repeat the cycle.
4) Mysterio his AAR is very nice when fighting champions like Domino, but she can still evade you. Now if you wanna use Mysterio against other mutant champs like Onslaught, Jean Grey, Cassandra, etc. you are out of luck.
How I would change Mysterio:So I wanna propose some changes for Mysterio that aim to fix the issues he has, make him meta relevant again while also respecting the character from the comics AND Kabam their design philosophy. That is why I propose the following kit:
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Mysterio his special helmet provides him the following benefits:
-Immunity to poison effects
-Immunity to ability accuracy reduction
-Immunity to reverse controls
-Incoming neuroshock effects suffer -50% ability accuracy
When Mysterio is inflicted with an armor break debuff or knocked down by a #Spider-verse Hero his helmet goes on cooldown for 10 seconds before reactivating.
Landing a second Medium attack generates 1 Chemical Gas, stacking up to a maximum of 3. Gas can't be generated while his helmet is cracked. Chemical Gas allows Mysterio to do the following:
-Landing a 4th light attack on a defender will remove up to 1 prowess effect from them per Chemical Gas on Mysterio.
-Landing a heavy attack will place a poison debuff on the opponent per Chemical Gas active on Mysterio, these poison debuffs deal direct damage over 8 seconds. This scales with Mysterio his attack rating. After using the heavy attack all Chemical Gas charges are removed.
-Mutants & #Spider-verse Heroes cannot evade while suffering from a poison effect.
-For each charge of Chemical Gas on Mysterio, increase his power rate by a flat 10%.
Using the special attack 1 will place grant Mysterio a Reflection Gas Passive lasting 4 seconds, if Mysterio has at least 1 Chemical Gas on him, consume one Chemical Gas charge to double the duration.
If struck while the Reflection Gas Passive is active, 70% of all damage taken is reflected onto the opponent when the passive ends and Mysterio heals 100% of the damage taken. Additionally, the first time the special attack 1 is used in a fight, Mysterio gains a Plot Twist effect. When Mysterio would be knocked out while the Plot Twist effect is active, he consumes the effect to provide him a 8 seconds long indestructable passive.
Using the special attack 2 will gain Mysterio an Illusion Gas Passive lasting 6 seconds, if at least 2 Chemical Gas charges are active on Mysterio, consume 2 Chemical Gas to double the duration of the Illusion Gas Passive.
While the Illusion Gas Passive is active, Power Drain 6% of the opponents max power per second and incoming attacks have a 25% chance to miss. This miss chance is increased to 100% while charging a heavy attack.
Using a special attack 3 will gain Mysterio a Frenzy Gas Passive lasting 10 seconds, if 3 Chemical Gas were active when using the special attack, consume 3 Chemical Gas to double the duration of the Frenzy Gas Passive.
While the Frenzy Gas Passive is active Mysterio gains a huge boost in Attack Rating.
Mysterio his signature ability generates 1 Chemical Gas every 6 seconds while his helmet is active. Additionally if Mysterio is already at 3 Chemical Gas when this would trigger, inflict the opponent with a poison debuff with half the potency of his heavy attack inflicted poison debuffs.
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Explaining the changes:So just to quickly summarize what changed in this kit compared to the kit he currently has in game:
1) While the helmet is active, Mysterio is immune to reverse controls
2) While the helmet is active, incoming neuroshock effects suffer -50% ability accuracy
3) While suffering from a poison effect, Mutants cannot evade Mysterio
4) Chemical Gas charges on Mysterio boost his power rate.
5) Using a 4th light attack no longer applies a passive heal block, instead it removes up to 3 prowess effects of the opponent.
6) Special Attacks 1 & 2 now consume less Chemical Gas charges and gain a slightly longer duration.
7) Using the special attack 1 for the first time in a fight gives Mysterio a Plot Twist effect, this makes it so that the first time you would be knocked down you gain an 8 seconds indestructable passive.
8) Added an effect to the signature ability making it so that when Mysterio would passively generate a Chemical Gas while at max charges, he now inflicts a reduced potency poison effect on the opponent.
Looking back at my earlier set out goals for this buff, I think I managed to succeed in all my goals.
1) Mysterio respects the class wheel --> Cosmic champs are often immune to poison, have armor breaks or both. This allows them to ignore most of Mysterio his defensive tools. Meanwhile mutant champs like Jean Grey, Onslaught & Cassandra Nova are a lot easier to take with Mysterio due to the reduced AAR on neuroshocks and his reverse control immunity. Additionally he shuts off mutant defenders their evade while they are poisoned so he can also easily take fights such as Domino or Nightcrawler.
2) Mysterio gains a boost in power rate & uses less Chemical Gas to activate his utility, this should make his rotations quicker. Also since he now inflicts 3 poison debuffs on the heavy, instead of 1 poison that is 3x as potent he can more easily control healing opponents. So he no longer needs the heal block on the 4th light attack, instead he can now control prowess on the opponent with it.
3) Mysterio still is a strong defender, he is slightly stronger too due to the signature ability change and the Plot Twist passive off the sp1. Both these effects feel very in character as Mysterio has faked his death many times in the comics. This however doesn't make him that much stronger on defense when using many of the meta relevant cosmic champions.
So there you have it, 8 changes to Mysterio his kit that would help make him more meta relevant, respects the class wheel, keeps his comic lore in mind and should not be too difficult to implement as it keeps most of his current kit in mind and all the changes use mechanics already in game such as removing prowess or the new signature ability which was inspired by the Act 6 Mysterio boss.
I hope you all enjoyed this buff concept, took me a long time to write it but it's worth it for my favorite fishbowl wearing illusionist. Have a great day all!