The next 4* character has been revealed: Mockingbird (Bobbi Morse). She can be seen in-game in the event preview for the Simulator story event, and there is a Piecing Together Marvel Puzzle Quest article on Marvel.com officially announcing her. (The Piecing Together article also revealed the next story event following Simulator: Meet Rocket & Groot; the Upcoming Events page has been updated accordingly.)
Mockingbird looks to be a support character with a Red/Yellow/Purple color set. Here are some descriptions of her powers at level 70, with 5 covers:
Bombshell – 9 Red AP
Mockingbird sets up a bomb and walks away without looking back. Creates 2 2-turn Fortified Red Countdown tiles. When one expires, if another exists, remove it and deal 6594 damage. If not, deal 5001 damage to the target and 649 to Mockingbird.
Mockingbird deflects attacks with her battlestaves, providing tactical openings for her team. Whenever the enemy makes a match-4 or greater, if you have at least 6 AP in that color (including Team-Up), fire a random friendly power at no cost. If not, gain 3 AP in that color.
Secret Mission – 8 Purple AP
Mockingbird steals enemy intel to gain an advantage. Destroys up to 8 AP of the enemy’s strongest color and generates 5 AP in your strongest color. (This power’s cost can’t be reduced.)
(Passive) As long as you have at least 5 AP in your strongest color, friendly matches deal 229 more damage.