Hmm... sounds good for a start, and the different characters does help to keep things interesting. But I have to say, I still can't really visualise how the game will turn out for now. As such, it would be best if you can provide a simple drawing of how the environment world will look like.
As for the speed, is it necessary to give control? I'm thinking that perhaps you can make it not controllable, and there will be power-ups available in the level that speeds up their speed, making it even harder to control. This adds difficulty to the game which may make it more challenging for players. Just a thought.
Hope all these feedback are useful, and do return the favour by commenting on my game idea as well!
Link to my game idea:
https://forums.tigsource.com/index.php?topic=66024.0Cheers,
Tereize~