There are sooo many, but one at a time will have to do. Good news to see the DH4 following the Halberstadt CL.II so closely.
The strategy of filling in mid-late war before going early is not a bad one. I think that there is a bit more to do in that time period before moving on. The Germans could definitely use a late war recon like the LVG C.V. The Salmson would make a nice addition for the French and Americans. The British are doing quite well with the RE8, Brisfit, and DH4.
Going backwards from early 1917 ...
Sopwith Strutter (used in numbers by French and British)
Dorand AR - this is a tough sell because it is not well known and utterly un-sexy. It was the bridge between the pushers and the later war models, so it is important.
to late 1916 ...
So we're less than 10 planes away form a nice stable of 2 seaters and really only 2 away from late 1917 to end of war.
Rumpler C.IV: Nice but kind of dull - circle until you get so high that nobody can catch you - fly mission - return
Aviatik C.III: Stable mate to the Alb C.III.
Albatros C.V/VII: Stable mate to the DFW.
Albatros C.X/XII: Nice variety for late war.
Hanover CL.II: Stable mate to the Halberstadt. Iconic Schlasta aircraft.
Voisin L.III: Stable mate to the Farman.
Caudron: Iconic early war AC used for a variety of mission types.
AWFK 8: Later stable mate to the RE8.
On the scout side the Halberstadt D.II would be great, as would the Morane N for early war and the Nieuport 24/27 for mid war.
etc. etc. etc. Coming along nicely.