I think once ATO is in place, and the question marks on the southern half are resolved (Maidstone) - any growth would need to be with narrowing the gap between peak and off-peak 24tph. The Finsbury Park services are, I believe, all covered now - I don't see Welwyn-Maidstone happening.
Growth therefore could be in more slows (all stations to BXT or St Albans/Luton) - or in whichever stopping patterns are chosen for the outers.
Clearly, 'fast to St Albans and all stations to Bedford' is not suitable for all of the South Midlands discussions, especially with new stations, now wires to Corby, and hopefully one day beyond M H'boro. Leicester is 100 miles from London and further than Thameslink should really extend - but 1tph EMT electrics might suit instead. It's at least a destination, no offence to Corby but I can't see much counter-travel.
I think anything on the LO network to Dudding Hill should be completely separate, and doesn't substitute TL Core commuter demand remotely.