^{I want to start this off by stating that the following statement reflects solely one player's opinion and experience, considering all available data I have currently.
I am by no means the sole authority of all that is Marvel Rivals, nor the best player in the world.
I welcome critique as long as it is conducive to the discussion, so have at it!}

^{With that being said, Keep on battling, and may your strategies always prevail!}

Mr. Fantastic

From my observation, I have to call into question Marvel Rivals logic when creating the kit for Mr.Fanstastic for the following reasons.

C͟C͟
I mention this because he possesses three distinct types of crowd control from just two abilities.
Distended Grips can pull 1-2 targets and knock them airborne, while Brainiac Bounce can AoE damage multiple targets with a slow that reduces by 10% per leap, stacking up to a maximum of -60%.

H͟e͟a͟l͟t͟h͟,͟ S͟h͟i͟e͟l͟d͟s͟ a͟n͟d͟ D͟a͟m͟a͟g͟e͟ M͟i͟t͟i͟g͟a͟t͟i͟o͟n͟
Reed Richards aka Mr.Fantastic has a baseline HP value of 350, making him the second DPS with the highest base HP, with Black Panther coming in at 300 Base HP.
The fact that not all Duelists share the same HP is wild, ranging from 250 to 350.
Now, if we factor in Flexible Elongation which gives him an additional HP Shield bonus of 75 which has a mere 8-second cooldown, it will bring him up to 425 HP and give him a free Dash.
Additionally, if Mr. Fantastic utilizes Reflexive Rubber, granting him a 300-damage shield and achieves full Elasticity (from Elastic Strength), he consistently maintains a staggering 1150 soak pool while getting increased damage as a Duelist.

To preface this, I want to put a start on the following sentence, because some Vanguards have the means to grant a greater HP pool than Mr. Fantastic.
I'm including this to highlight how Mr. Fantastic, despite being a Duelist, excels in both tanking and dealing high damage, often outperforming most Vanguards in these aspects.
If we look at the Vanguard role, we see that Mr. Fantastic will have a greater HP pool than any of the Vanguards while they are at base HP pool value.
For the record, I will not include Hulk because Hulk is a special case.

For reference, this is the following base HP pools for Vanguards:
Thor (625 HP)
Magneto, Penni Parker, Dr. Strange, and Venom at 650 HP
Captain America at 675 HP
Groot at  700 HP

T͟e͟a͟m͟-͟U͟p:
While I believe Team-Ups are an interesting part of the game, I do not believe that in the current state of the game, they are implemented or balanced properly.
Considering this, if Invisible Woman and Mr. Fantastic are on the same team, they benefit from increased damage resistance and ongoing generation of Bonus HP.
This resembles adding a mountain of crystal sugar on top of a lava cake—it's simply excessive.

In summation:
Based on my observation, Mr. Fantastic presents a significant balance concern. He combines high DPS capability with exceptional durability, thanks to layers of crowd control and damage mitigation/amplification. 

Scarlet Witch

Overall, the Scarlet Witch is generally acceptable in most cases.

However, the inclusion of a laser that requires minimal aiming—merely pointing your reticle and locking on—may not align well with the gameplay of Marvel Rivals.
It is also worth citing that the laser has a radius of 3m with a range of well over 20 meters!
Additionally, the ability to apply a permanent slow effect over a considerable range raises notable considerations regarding game balance and mechanics.

Drawing a comparison, Symmetra from Overwatch faced similar scrutiny for her auto-lock mechanics, which were eventually adjusted after several years while retaining the basic functionality.

They could benefit by following suit regarding Scarlet Symmetera- I mean Scarlet Witch.

Once again, good luck in your games, Heros!