I do think the Croydon / Gatwick / Brighton arc is the only one south of the river which would be viable for service from north of London.
Where 2 or 3 worthy destinations lie in a nice succession, I'd say the next best group is Woking/Basingstoke/Winchester/Southampton, or Woking/Guildford, but the former is a much longer journey, and XC covers this. I guess this would also run via Clapham Junction. Windsor Lines are easier though, but even fuller.
I also doubt high frequency demand north of Milton Keynes, and I'm even skeptical there. Are the good people of Tring or Hemel going to Gatwick often? MKC itself would still be best off via a fast to Euston.
The very first thing for any growth or success would be addressing clean access from the WLL to the Brighton Fasts, be it a dive-under somewhere around Clapham Junction or another way. Otherwise the service remains local.
So, and using platform 10 at Watford:
>> infrastructure as today:
2-4tph Watford Junction - WLL - Clapham Junction (run by LO) - paths towards East Croydon released to more Victoria trains.
>> with Brighton fasts access / dive under:
2tph Watford Junction - WLL - Clapham Junction - East Croydon - xyz pattern to Brighton. If popular, extend to Milton Keynes, especially once EWR opens with a Bletchley call.