Meteor/Vampire Mystery Update:
1. The missing Meteors
-- The current heavy line is composed primarily of two-seat twin-engine aircraft and circumvents the Meteor by way of the Westland P.1056 and Gloster P.228 (a Meteor analogue).
-- Preliminary work indicates that another heavy line composed of single-seat twin-engine fighters is possible from Tier IV to X (Gloster/Westland).
-- It is possible to use the Gloster Meteor development in jet form as early as Tier VII (as indicated by Meteor Mk.I premium), and as late as Tier X (as indicated by engines evaluated on Meteor airframe).
-- the decision to use Gloster Javelin as the Tier X in the current heavy line might suggest that the Meteor (or some form of it) is to be taken to Tier X, or that the line might transition to the English Electric Canberra (designer previously worked for Westland). Given the Meteor's use in a ground attack role it would not be unreasonable.
2. The missing Vampire/Venom/Vixens
-- Vampire/Venom/Vixen were also circumnavigated, so the question is where they fit in relative to Meteor, and whether or not a line of development is possible or necessary leading up to them at Tier VIII, IX, X.
-- There are essentially 2 ways to complete a De Havilland line -- on the basis of engines (Napier piston engines) or role (carrier-based strike fighter):
1) From an engine development point of view, the De Havilland jet engines (Goblin, Ghost) were developed by Frank Halford. Halford's career as an engine designer did not begin with jets, but rather began with De Havilland Gipsy, Gipsy Major, and continued with Napier & Sons in the development of Napier Rapier, Napier Dagger, Napier Sabre. The Sabre engine was most famously utilized in the Hawker Typhoon and Tempest, but Napier engines were also used by the De Havilland DH.77, the Martin-Baker MB.2, Martin Baker MB.3, and Blackburn Firebrand. (Martin-Baker MB.5 deviates by using a Rolls-Royce Griffon but it is a remarkable aircraft without a home if omitted on that point alone).
2) From the carrier-based-strike-fighter point-of-view, the line becomes more flexible about what it can include, but then it becomes very difficult to provide a "uniform feel" to the line. This method might include Fairey/Blackburn fighters and/or light bombers.
3) some combination of the above
-- If Vampire/Venom/Vixen are instead added as a minor branch, then it would make the most sense (in my opinion) as a minor branch coming from Hawker Tempest (based on the Napier engines also designed by Halford).
3. Moving forward
-- resolving De Havilland / Airspeed ... possibly augmented by Martin-Baker / Fairey / Blackburn
-- accounting for alternates
-- filling in details