I think it’s been given a cursory look, but it’s riddled with issues, not least that it was never double track, there’s Watford North Level Crossing, and any freight would have to go to/from the south.
Well the first thing is to identify a supported traffic need and funding.
A bit challenging - but much can be done with the above , plus the necessary consents , environmental protection and so on.
Issues include restricted access to the WCML - now on the "panel" but there are long track circuit constraints for moves on / off the branch via the slow lines. (basically an up train needs to be clear of Bushey platfrm before another train can be accepted.
Watford Yard has more going on these days - aggregate terminal for one and there are ideas / aspirations for major brownfield development.
There was "double track" to Watford North for carriage sidings etc and various factory sidings at one time (including a wafer biscuit plant !) - space for double track beyond Watford North crossing almost to Garston (site of the old coal concentration depot - now "woodland")
A new formation is needed from Garston station to Park Street - including going under the M1 and across the M25. The former probably OK - the latter needs a new bridge span. Of much more concern is a particularly sensitive stretch of attractive (old) woodland , Bricket Wood station - "the common" and picking a widened alignment through what are now quite well populated suburban developments. There are of course hopes for the new loop at Bricket Wood.
Interfaces at Park Street - a location already more than aware of development. Well organised I should think.
Of course , all of this - and not affecting the MML is a "win" - and it would link into a gauge cleared WCML South - but we all know how challenging that is for capacity and performance - (yes HS2" etc) - but the value of a Watford - Wembley path even post HS2 compared to the same path extended to say Daventry (to be fair and to quote another freight destination is a debate to have) - I would expect the M25 crossing would be a superb little job for an up and coming civil engineer , or even a seasoned one.