[Not2B]

You can get the shafts and shaft collars from McMaster Carr - of course I don't know what the rules will say about these parts - but they do work.

I found that I ran out of shaft collars really fast.

Other than that I'd say a few extra motors and some more metal - but it depends on what you want to do with the kit. (Motors and metal is just what I would want.)

Good luck! Vex is fun to play with. (The dog doesn't agree.)