I think this would be great. It would also be nice if you could choose multiple jobs/roles for an instance. Like if I want to do Titan HM, I could choose PLD, BRD, WHM. Then whichever one is needed is the job I'm assigned.

This would make the DF process much more complicated, but maybe they could keep it the way it is for a quick/easy DF, but have an Advanced link that would open up the additional options (select instances and the jobs you are willing to play for each one).