Depends on what you mean by "fundamental". As I see it, the relevant
questions are:
1) do 3D models require different "hardware" support or support from
the transmission medium to be usable, from (say) that for image/* or
application/*?
2) is there a default user agent behavior that applies to *all* 3D
models in your proposed world/* class?
The top level in MIME types is designed to allow filtering by gateways
between different kinds of messaging systems (so that you don't try to
ship audio/* to a pager or image/* to voice mail), and also to provide
default user agent handling of content-types. So the argument needs
to be pitched in these terms.
Keith