While Helga first made a lot of sense, and I definitely respect that choice, I would still like to see Arnold in at some point. He's way too important, he DOES have plenty of moveset potential, and he has loads of fan demand. Gerald's fun but not really a character people are asking for.
As for move unlockables, if we get that feature in the next game I'd love to see Danny and Lincoln get some new moves. Danny has loads of unrepresented abilities to choose from, and Lincoln could easily tap into more Ace Savvy moves, his ARGGH! gadgets, his magic skills, his David Steele stuff, etc (hell, his grandfather's girlfriend recently taught him how to do an epic dodge roll!).
Him being an everyman isn't really the issue here. I'd say it's more to do with the fact that they made the main character an advice-giving wisdom pacifist. I loved Arnold as a character, but that archtype doesn't leave as much room to get spicy or dangerous since there's a reputstion to uphold. Helga has a lot more freedom because she has a more flexible archtype.