I've only been using Carrara Pro for a few months now, but one thing I would like is the plant (tree) room to have more options to create flowers and other plants  and grasses (which can blow in the wind).  A really cool and easy use hair lab (one that is way better than Poser -not that Poser is bad but compared to some others it lacks a lot).

Maybe the programers could include a few premade scenes showing off the particle generator with examples of snow, rain, and fire so that really dense people (mainly me) can see how it works and try and duplicate it.

Volumetric material shaders, similar to the ones in Bryce because volumetric shaders are not just for clouds.

Maybe a wings3d plugin, or better wings 3d importing support.

Maybe photoshop like layer support for complicated 3d scenes.

More presets for the terrain generator, like icebergs, volcanoes, floating islands (Laputa), swamps and marshes, stalagemites and stalagetites.  The ability to hollow out terrains to make caves.  (I'm still having trouble adapting from Bryce).

Sincerely,

David  :p