You'll never ompletely replicate the current fares structure with contactless. Most people using contactless won't be aware of the fares they're being charged anyway and are highly unlikely to be swayed by a slightly differential fare on a particular route.
That said, every possible journey will have a default route. If desired an alternative route can be determined either by a pink validator (for a cheaper fare than the default)or by touches at an intermediate barrier lines en-route when changing (for a more expensive or cheaper journey).
In the specific case of Cambridge you could charge a more expensive fare by changing at Kings Cross or St Pancras or a cheaper fare by changing at Tottenham Hale or Liverpool Street. I suspect that only cheaper fares to Liverpool Street would exist in reality, fares to an underground station would probably be the same whichever route was taken.
The other thing to consider is daily caps as I don't think there's any facility for a differential cap delending on the route taken.
