Getting to Cambridge and Bedford from Brighton is pretty decent. Peterborough could be there in theory, isn't due to the service pattern.
I would think, if Cross County destinations were of interest, that extending a future wired North Downs fastest service from Reading might be worth trialling. Brighton to Guildford may well be interesting too.
The cross-overs between fast and slow mean a Brighton-Watford connection is likely to never return. But Watford is not what it once was, for WCML connections - those may return though post-HS2.
Equally, I think you'd struggle to justify trains of less than 8 cars getting a path south of Three Bridges! And connections are pretty solid today, from a good number of stations.