Very cool idea, I feel your dream core! And I love your idea about the three characters having three different movement settings, that's very clever, but I found that there are times when tasks can be accomplished by cheating, for example, the suspended ladder requires three characters to work together to get to the top, but the Slime can just jump and go straight up, that sounds unreasonable, maybe adding some hidden collider would be much better.
In addition, the size and style of the sound effects need to be balanced, because of the overly loud shooting sound effects will make player OFC.
Additionally different characters can add their own mini side quests while working together to complete a single quest, which this time gives a positive feedback mechanism to players who choose different characters.