Just to float a couple of things...
It would be good to have a simple, "dumbed down" career mode...
You "login" to vdrift and have access to whatever you want (cars, tracks, variants, whatever)
You're "history" of fastest lap times with which car stays with your career. A "history" of the car's evolution would be nice too - but not required.
I'm leary of the whole, "need to perform/do X to obtain Y". Some of us would like to just pick up a car and drive when we want - but still want the statistical part of a career mode that "free driving" is unlikely to provide.
I know that I don't have the time to sit in front of vdrift to travel the entire "career arc" and I'm sure other casual vdrift people out there would be frustrated by this as well. If I want a Ferrari I'll go "buy" a Ferarri independent of whether I've driven enough in a career mode (for example).
The other thing that I'm finding is that I'm using vdrift to tune/modify my virtual car with an eye on what it would affect on my real car (torque curve shape, diff ratios, corner weighting, etc., etc.). Sure, it's not an exact science but I think that vdrift/vamos is accurate enough to spot a trend.
I wouldn't want career mode to get in the way of this type of "tuning". A "simplified" career mode would be able to track this.
Just some ideas/comments.