I seem to recall the last plan was to run a six-minutely service between Clapham and Shepherds Bush. That would probably be a half-hourly "Cross London" from Croydon-ish to Watford-ish, and then the other services would all be London Overground, so there'd be a gap for a half-hourly Richmond to Stratford for that whole section to be every 6 minutes from Willesden to Stratford at high peak. That 10tph runs now, and with a couple of minor adjustments, you could incorporate a third and forth Richmond per hour into Willesden which then goes onto the Kensal Green turnback, and allows a half-hourly slot for freight and other such traffic between Camden Road and Lea Jn.
You'd have a bit of a struggle to accommodate that at Clapham, unless some LO trains run off Platform 17, and again a bit of a tweak which would mean some trains arrive a couple of minutes later and the like. But overall, it's possible. There'd also be a bit of space somewhere for some freight between Mitre Bridge and Longhedge / Latchmere.
A very standardised six-minutely (or very close) service from Clapham to Shepherds Bush is exactly what is needed. How you deal with the Southern service is totally different gravy. In theory, post-EWR, you would probably have a very strong business case to extend that all-day to Bletchley every 30 minutes, using the back platform, and that would leave Platform 2 / 2A at MK free for EWR trains coming onto the slows at Denbigh Hall.
Depending on devolution, you may be able to find a sensible extension of that service south of Croydon. If you assume Caterham, Coulsdon Town and Tattenham Corner are out (in case they end up being Overground), and if you assume Reigate post-CARS is a Thameslink service (replacing Rainham), and that stuff via Redhill and Horley needs to be big, 12 car 700s, then you end up at extending them beyond Redhill to Tonbridge. That, restoring a half-hourly service on the Redhill - Tonbridge, bringing back Tonbridge - Croydon direct connections, and even adding in the option of a few extra stations at the 'busy' places like Balham, would make it a very good service.