Something i would personally love to see in the DLC..
Be able to customise veichles that you find around Liberty City, and store them in a Warehouse of some sort.
Think of the Saint's Row Veichle Customisation. I absolutley love it.
Maybe even to take bits off a few cars here and there and add them to another
And to finish off. Imagine being able to sell the cars online and be able to choose one from your online garage to race in or use in a free mode