I agree that Luton & stevenage need to be linked. As part of the project if feasible there needs to be a proper station built at Luton airport (albeit in a tunnel underneath due to the hill!) and direct access to the airport/Stevenage from MML both north and south.
Some Thameslink semi-fasts could run via MML & the airport to Stevenage & terminate at Letchworth (or on to Peterborough or Cambridge) to give access to St Albans etc
It also gives Stevenage 3 routes into London - Thameslink can use any of the 3 from the St Pancras low level platforms, so possibly partially relieving pressure on welwyn if say 1 semi fast ran via St albans? & allows limited diversions via MML as the Hertford loop cannot cope if welwyn route is blocked in the peak. Creating some more capacity via the airport as aprt of the EW link way probably be more viable than building a new viaduct to add 2 more tracks at Welwyn.
North Herts also needs access to Milton Keynes for WCML connections
I would enviasge a long distance limited stop say Norwich-Bristol service (probably taking over the extisting Cambridge-Norwich paths but limited stop at main stations (Stevenage, Luton Airport, Luton, Bletchley, Oxford, Swindon, Bath Spa)
Local stoppers: Cambridge (or Letchworth) -MK (or Rugby via Northampton).
Later phase - new link from Stevenage via Hertford to give access to Harlow & Stansted.