Let me start by saying overall I like the picture. You have good proportions, and excellent textures. The clothing looks good, but I think could be better by showing more folds. This would compliment your smooth style by adding some contrast and thus giving the picture some additional layers.
I cant stop feeling like this is a picture I have seen a hundred times before. I would have liked to see some additional originality. I know this is harsh, but it just seems like something that has been done but in your style.
Let me close by saying that I also like to take a concept and put my spin on it, and there is nothing wrong with that.
I always like to end my critiques with a positive. In other pictures I have seen like this (young xmen) the toons look to be the same age as they are in the present. Your picture does not do that. The toons look like they are late teen, early 20s, and thats how it should be.
I love the colors that you've used, especially for the background. it's very pretty. I also want to say that you are a very, very good colorist.
The only thing I feel is off is that you used roughly the same color value/strength and saturation for everyone. By doing so, you've given the image less depth than it should have had, and you've given the aforementioned characters less separation in planes. I like to break planes down into three categories. Plane 1 is the foreground, plane 2 is the middle ground, and plane three is the background (of course, when it comes to coloring, where backgrounds are concerned, you can have anywhere between 2 and perhaps 5 planes or more, but three is a very effective number of planes).
There should exist the darkest values and full-strength colors should be in the foreground, lighter colors and values in the middle ground, and the lightest colors and values in the background. You neglected to do this.
I have to use the disclaimer that I'm not a colorist, but had I been been told to give the characters separation, I would have went with three planes, with either one of these two routes:
Route 1: - Plane 1: Cyclops - Plane 2 : Jean Grey and Beast - Plane : Archangel, Iceman, and Professor X
Route 2: - Plane 1: Cyclops - Plane 2: Jean Grey, Beast, Archangel, and Iceman. - Plane 3: Professor X
By doing that, my eyes, and pretty much everyone else' eyes, would be drawn to Cyclops first, then who ever was included in plane 2, and then to whoever was included in plane 3.
Depth and separation is a powerful tool. Don't neglect to use them at your disposal.