Currently downlinked via Mode S from suitably equipped aircraft are selected flight level, indicated airspeed, heading, groundspeed, rate of climb, rate of descent and flight id. Coming soon is set barometric pressure setting.
If I may play the cynical devil's advocate here, perhaps I am being naive but could it not be said that:
1. Selected flight level is the job of the crew and not a matter for "checking" by ATC. Actual flight level is of course already available from Mode C.
2. Indicated airspeed cannot be relevant to ATC apart from again "checking and policing" purposes. Ground speed is what they are interested in for planning purposes and presumably is already computable from existing radar rate of change of position information.
3. I don't understand what advantage heading info would be over track info other then to save a few micro seconds during vectoring. Track info is already available from conventional radar.
4.Groundspeed must already be computable from existing systems.
5.Rate of climb and descent must already be computable from rate of change of altitude given by existing transponders.
6.Flight ID is useful for policing but not relevant to traffic control.
7.Barometric pressure setting is again a matter for crew and not ATC checking and policing.