I do wonder whether you could free up paths by combining the Littlehampton and Eastbourne services again, with a split at Haywards Heath, and removing altogether the Three Bridges (?) terminating Thameslink, transferring its stops to the Peterborough-Horsham so that this became a simple all-stations stopping service beyond Purley.
Might that lead to a possibility of Clapham Junction and East Croydon stops on the Brighton fasts?
I always thought that the Gatwick Express lost its raison-d-etre when the 73s and IC stock ceased to exist. In the days of the 73/IC sets, you could get away with charging a premium price as it was IC-grade stock. After the 73s, it was basically just another service, but priced at a premium. I always thought it would have been better to replace it with a regularly-priced crowd-busting shuttle from Victoria to Gatwick calling at Clapham Junction, with the aim of keeping Gatwick passengers off the longer-distance services, reducing overcrowding.
Perhaps in an ideal world we could have something like (timings only examples, not concrete suggestions):
xx00 and xx30 Brighton fasts calling at Clapham Junction, East Croydon and Gatwick (the East Croydon call being to provide a regular fast service from there to Brighton)
xx15 and xx45 Gatwick 'crowd-busters' calling only at Clapham Junction (East Croydon having plenty of alternatives)