*jumps into discussion randomly and uneedingly*
I think it'd be cool if we had a few levels where the difficulty changed depending on what character you had. Like it'd be the easiest to beat with the set's character, and progressively harder as the characters became more unfit. I mean how easy would it be to beat a water level with the flight character, or some other unwieldy combo? I mean, we don't need separate paths for them, they'd just have to make due with the extremely unfitting circumstances. It'd be like a water-level in HATIC that centered around Armin needing to swim. And these wouldn't need to be mandatory, but at least maybe offer a reward or challenge to the hardcore players.
*jumps out of discussion*

That'll be cool... but it'll be difficult to make the levels fit with all characters and stay all complex.

Okay, I'm finally taking the time to catch up on this conversation so let me reply to some old posts first.

I think it's odd that you're introduced to the game with Hugsy, and then you basically never see him again. Players will think Hugsy is the main character or something, and throughout the whole experience they'll be expecting him to show up again any minute now. While this might make it more exciting when you finally find him again on the moon, it's really something that irritates you the whole way through. Furthermore, we don't even know if players will remember who Hugsy even is by the time they find him.

The problem with this is that we might be introducing the player to too many characters at once. I think it might be better to have them play maybe 2 or 3 levels with a character before moving onto the next one, and then they'll play a few levels with that character, and so on. We don't want to create a feeling where we're front-loading all of these characters and their mechanics. I think it's best if we introduce characters gradually throughout the experience.

I got an interesting idea. We could have a SMW-like map where you watch the world grow and change as you beat more levels, and in order to unlock new characters, you must "find" them in the world map, by unlocking new regions (each region introduces a new character). This forms almost a kind of secondary game where you're trying to figure out how to expand your map (by finding the secret gems or exits in each levels), and you get rewarded for making progress by getting introduced to new mechanics. Of course, the biggest problem with this idea is scope. It really demands that we make a lot of levels.

I'm thinking we have way too many characters for the ideal scope of the campaign. Maybe we should cut one or two out?

On the topic of whether or not to have a linear campaign, having the freedom to select from a number of new levels is great for when players get stuck on a challenging level, since this allows them to continue playing the game. The challenge is how to balance this freedom with a good difficulty curve. I think we should look at how other games have managed to do this in order to get a better idea. If I remember correctly, N did something like this.

N had 100 levels (I'm pretty sure) and they broke them up into 20 or 25 level sections.  You could play the first level of any section, and beating that level would unlock the next level in that section.  So yes, they did what we are thinking about doing.

I wish all levels have some unique gimmick... it makes it interesting yah know?

For the record, N had 500(!) levels. They were split into 10 different columns with 10 "episodes" in each column, each episode containing 5 levels. You started with the topmost episode in each column unlocked, and unlocked the one underneath after beating the previous episode in one go.

I agree with cutting a character. 3 is probably optimal rather than 5. 4 is fine.

If we cut two characters, then I suggest cutting two of Ivan, Jean and Rita. I think Rita would be hell to control, so I doubt anyone would like using her. Ivan is only really suitable for water levels, which are generally the most hated levels in any game, so I doubt he will get much use either. And Jean just doesn't seem unique enough. We already have Hugsy and Floyd for generic characters. Or we could cut all three and add a badger or bear "power" type character, who could push boulders without slowing down much and have less recoil when breaking through crates.