I think it was the first DLC I ever bought. I really wish they would make a Crackdown 2 although I believe it is extremely unlikely.
It's a no go from Real-Time Worlds. They wanted to make a sequel and left it on the table for Microsoft, Microsoft took too long and Real-Time Worlds had already started development on APB and another game (as of yet unveiled), so it's not impossible, but much like PGR now, it won't be buy the original developer.