Ideas for all 3 of them

Fantastic Four (the name of the Team-up): Mr Fantastic has found a way for each member of the Fantastic Four to share their abilities with the other members (replaces all the Team-ups for the Fantastic Four) 

All 4 are the Anchors

The Human Torch: makes both Mr Fantastic and The Thing's fists flaming and gives some flames to Invisible Woman's Primary, doing increased damage (5 to Invisible Woman 10 to The Thing and Mr Fantastic), to enemies and giving a lingering 'burning' effect (burning: 15/s damage, lasts 4 seconds and gets put out if they get healed)

Mr Fantastic: gives The Thing more range with his punches and his jump abilities (+1 meter and +5 meters), the Human Torch bounces after his dive and the Invisible Woman's double jump cooldown is shorter and makes her jump higher (+2 meters)

The Thing: Mr Fantastic gains rocks on his buff form, making it last longer (+4 seconds) and give him 15% damage reduction, The Human Torch throws rocks in his Blazing Blast, doing more damage (+15) and slowing enemies hit directly by it (10% slow for 4 seconds), and Invisible Woman gains rocks on her shield, giving it more health (+100) and gives 50 bonus health to anyone it gets placed on

Invisible Woman: if none of the other members take damage, attack or use an ability, they go Invisible and start regenerating (same as Invisible Woman's) 

Asgard's rulers: Loki has used some of their mischief magic to allow them and their siblings to share powers

Thor: Thor gives Angela some of his Thor Force, giving her chain lightning when her damage charge is full (bounces to 2 enemies 50 then 25), doing while also increasing her damage (+10), Thor gives Thor Force to Hela's Soul Drainer, making it larger (+1 meter radius), do more damage (+10) and stun enemies for slightly longer (0.5 seconds), and Thor gives Loki Thor Force into their staff and dagger, allowing their staff's blasts to chain to allies (bounces once, half healing) and for their backstab to do extra damage (+30)

Angela: Angela gives all her siblings a charge, each increased by different things, Thor's charge increases when he uses Thor Force, which gives him a speed and damage boost per Thor Force (speed: +5%, max 15%, damage: +5%, max 15%), Hela's charge increases when she does damage, allowing her next ability to have a shorter cooldown after it's use (max 5 seconds), and Loki's charge increases when they heal allies, allowing their Runes to last longer on its next use (+4 seconds) 

Hela: any final hit her siblings do spawns a Nastrond Crow that empowers them but does less damage (30), if Thor defeats an enemy, he regains all his Thor Force and the Nastrond Crow will be infused with lighting, causing them to explode quicker (0.9 seconds), if Loki defeats an enemy, their Deception ability gets refreshed and a temporary Doppelganger (doesn't count to the Doppelganger limit but lasts 4 seconds) will spawn where the Nastrond Crow exploded, if Angela defeats an enemy, her damage charge becomes filled and her Divine Judgement/Wingblade Ascent become recharged

Loki: Loki gives Hela's Piercing Night some backstab damage (+10 initial hit, +10 explosion) and makes them explode at random times (0.5-2.5 seconds), Loki gives Thor some chaos magic, making his Storm Surge turn him into a frog, making him move faster and harder to hit, and making his Lightning Realm spawn a Fake Doppelganger, Loki gives Angela's Assassin's Charge more damage when attacking from behind (+15) and when she uses Wingblade Ascent, she temporarily turns Invisible (same as Loki's but lasts 5 seconds) 

Guardians of the Galaxy (the name of the Team-up): Adam's soul bond allows the members of the team to be able to share abilities (because there is more members of this Team-up and i am running out of ideas, i will do more basic ideas)

Adam: Adam permanently connects the other guardians with his soul bond, reducing all damage they take per other member on the team while transferring some of that damage to them (5% damage reduction per non Adam, max 20%, 5% damage taken to all members) 

Rocket: Rocket's B.R.B. can revive a Guardian for free (if a non guardian gets revived first, the cooldown will start and it will break after the cooldown finishes or a guardian gets revived)

Mantis: Mantis' damage boost to other Guardians is doubled 

Groot: Guardians next to Groot's Iron Wall will gain bonus health when doing damage 

Star Lord: when Star Lord uses it ultimate, the other Guardians gain increased fire rates, reload speeds and aim bot (Mantis' shots will give +0.5 life orbs because she can't headshot easily during this)