Creating an XML 1.1 format for 3D models

[eSite]

Hello,

I want to create an XML 1.1 format to store and exchange parametered primitive shapes of 3D models.

Here I'm not asking about the 3D part, rather I'ld like to know "what there is to know before creating a new format".
If you did that in the past, please share your experience.

PS : I'm still reading [w3schools.com...] and [w3.org...]

[cmarshall]

What is X3D [web3d.org]