I always favour the service style of where a singular city has 2 or 3 stops with the idea of 2 outer edge stations and the main central station. So Watford Junction would be the Outer Edge of London station and I'd have all trains which are for the Birmingham Cluster stop at Watford Junction. The Leighton Buzzard calls make almost £14m pre-covid so the level of service should be retained although with the extra capacity from HS2 it might make sense to move them to another service. Also LBZ to Watford Junction makes £519k so it is one of the best flows not involving London BR.
To be honest I think the present service is an absolute masterpiece of timetabling which (when considered with stuff like the Marston Vale) carries most of the benefits of Takt. Thus I'd keep it as the base and make only minor tweaks plus add stuff on top.
So perhaps for the LNR services we could have:
1. Make the hourly peak fast Euston-MKC-Northampton hourly all day
2. Add a WFJ stop to the other Birmingham service (providing for Leighton-Watford too)
3. Add 2tph Euston-Harrow-Bushey-Watford
4. Remove the Harrow and Bushey stops from the existing stopping service, speeding it up a bit
and 5. Reinstate the Southern service, ideally at 2tph.
That lot would give Watford 6tph to/from Euston which is really pretty good, as well as improving things for Northampton and not significantly disadvantaging many people.
One thing I would like to see would be the construction of a Marston Vale-MKC chord, which I'm pleased to see does actually feature in Network Rail documentation and not just my head. The line would be quite busy if it went to MKC instead of Bletchley and if the stations along it were used as the centre of small eco towns, and would be a decent start of an "MK Metro" (with apologies to Julian Peddle's old bus company). We could also have all East West Rail services run via MKC in that case, serving one of its primary markets. HS2 would release the paths to do this, though apparently 6 tracking Bletchley-MKC is under consideration as another way to provide them.
Pre COVID there was consideration of a much higher frequency on the south WCML, but I just don't think that's justified any more - extending all services in the above pattern to 12.350 or 10.730 would provide pretty much all the capacity needed for a long time.