The Xbox 360 DLC is obviously going to be all the features that are in the last two GTA games but missing in 4.
It's not that simple. I am sure a lot of the stuff that is missing could not be done for technical reasons.
I am fairly sure planes and fast bikes/cars could not be done due to them requiring the city to stream data faster than the engine could allow (the sme reasons they were omitted form Saints Row). Whilst it is conceivable these may be overcome by the next release (GTA4:xx) such major changes won't be in the DLC. I would place money on it.
rp2000