They've said in the last episode of 2KTV that we will be getting cards for challenges down the road.
I've not given up on challenges yet. They seem to have worked most of the kinks out of this mode. Maybe some more stand alone challenges to kick it off would have been nice, but overall it's more polished than most of the MT experience. And we are only 8 weeks into the game. I'm sure there's much more to come.