The problem with the next frequency is that it would need to be input manually into the system because depending on adjacent sector configurations at that point in time etc, the frequency could be different.
I think there is a slight misunderstanding here. The Mode S would send down the frequency which the pilot has selected, not what you want him to dial up next. Useful if you get a Loss of Comms (PLOC) since you know where he might be RT wise, but not much else.