Indeed. On a regional level, I think a Brighton-Lewes-Uckfield-Eridge-Tunbridge service would do well...but would need that proposed loop:
...though Ideally some way would be found to restore the long-lost route over Cliffe High Street.
If capacity on the Brighton line to London is what you want, you need to be competitive with the current BML, which going via Eridge is never going to be. Realistically, you need either more tracks on the BML itself, or a more direct route that largely parallels it. NR's proposed works at Windmill Junction will unlock capacity there, and if BML relief as a whole were pursued, then I'm confident a 6th track from South to East Croydon would be viable to provide the capacity needed there.
I'm almost certainly repeating myself (sorry!), but if you ask me, (and no-one will

) the obvious routing for such a relief route would be south from East Grinstead. There is a reason that railway was built originally, and serving new markets whilst relieving the BML was largely it. Yes, the Bluebell are an awesome organisation and have put so much into reopening the foolishly-closed route, but if they were suitably compensated I'm sure things could be worked out (NR funding an extension south from Sheffield Park, for example.)
Once you hit Horsted Keynes you have a choice: you either go south-west to Haywards Heath or south to Lewes. Lewes will give faster journey times, but a) you will be obliterating the whole of the Bluebell operation, and b) you won't have interchange with the BML, so you will be relying on passengers using the roads or residual shuttle services to intelligently balance loadings themselves on which line to use (which I strongly suspect would fail miserably).
Once you reach the BML, build grade-separation for Copyhold Junction as a mirror of what is planned at Keymer Junction to the south. You then have 4 tracks into Haywards Heath with cross-platform interchange.
Along with this, Network Rail are proposing one of the options for Keymer's grade-separation to be 4 tracks north of the flyover though to a point north of Wivelsfield station, with that station being rebuilt with 4 platforms. This leaves at most just a short 2½ mile stretch of double track (which could be managed as at Welwyn, I suspect), with the only two notable obstacles to full segregated 4 track section being the short 249yd Haywards Heath tunnel and the short 150yd viaduct south of there, both of which strike me as quite solvable problems.
Admittedly, this route is indeed slightly longer than going via Sheffield Park, but gives you interchange with the BML at Haywards Heath and Wivelsfield, so those wanting stations between Haywards Heath and South Croydon can still use the train from Lewes etc. without driving, which I feel is important, as the whole point is that most of the capacity would be routed via East Grinstead and not Gatwick.
IIRC, a service from Lewes to London would take ~4 minutes longer via East Grinstead than Gatwick at an average speed of 60mph. That seems to me to make far more sense than BML2 routing via Uckfield and the tunnel under the downs. It also enables you to make better use of paths by combining East Grinstead and Seaford/Eastbourne services. As I doubt the east coastway would be able to fill the capacity of a double-track line from Croydon to Oxted, you also gain the option of routing some BML express services that way via Copyhold (if they don't stop, the route they take is largely irrelevant!), freeing capacity further north for more services from the Arun Valley line (and depending on how you use the Quarry lines, the North Downs and Tonbridge lines) to join the BML.
Just my 2p.