I'd have to disagree, switching between characters at will is a key part. I cannot see this being doable online. I can only imagine one of these:
1] People will be stuck as one character in a mission. IE, you're playing Gauntlet/Robotron. There'll be a race to get the clear favourite and some will be stuck as characters they don't wanna play as.
2] People can choose which character to be (most likely from a small selection depending on the mission/map) and there will be a multitude of the same characters. Can work in Mario Kart, but I don't think it'd work in this.
3] You can switch but it would force the other player to your former character. Simply not going to happen as it will be a switch war.