As mentioned on Uckfield-Lewes, the Norwood/Croydon works will help to untangle and potentially identify new paths south.
A second train per hour via Uckfield would be a good contender, this could happily take over a 'Plumpton' service towards Eastbourne (see below on Haywards, but short answer, GatEx). If further demand was there, splitting or connecting shuttles to Oxted/Hurst Green could be explored.
Another beyond post Covid action for Three Bridges, Haywards Heath etc would be to finally kill the Gatwick Express, tweak paths and slots accordingly out of Victoria and through Croydon - and ensure that all four of those tph run to the Coast, all the time. That may allow some consolidation of similar services or diversion to other locations. Gatwick could also terminate some extras from the south, and offer a lot of onward options, in addition to being a destination itself.