Texture Atlas

In OpenGL, array texture extension is a great way to define a stack of 2D textures which can be useful for texture atlas.

The problem is that Collada specification only offers 6 types of sampler :

How about a new type : ARRAY ?

All of the sampler parameter types you listed can be in arrays in COLLADA FX profiles for CG, GLSL, and GLES2.
I recommend reading about the <array> element in the COLLADA 1.5.0 specification, chapter 8: FX Reference, and also all of chapter 7: Getting Started with FX.

Since you are interested in OpenGL, please refer to chapter 11: Types, section Parameter-Type Elements, regarding <glsl_value_group> that includes all the sampler types.

I agree 100% with you.

besides this issue, do you know if a new spec is coming in the near future ?

An image array can be represented by using the <array> element within the scope of the <create2D> element.