including some of the other participants in Great War:
Italy: Ansaldo A.1 Balilla http://www.theaerodrome.com/aircraft/italy/ansaldo_a1.php
Austria: Aviatik D.I http://www.theaerodrome.com/aircraft/austrhun/aviatik_di.php
Russia: Sikorsky Ilya Mourometz V http://www.theaerodrome.com/aircraft/russia/sim_v.php
& of course
France: Nieuport 17 http://www.theaerodrome.com/aircraft/france/nieuport_17.php
IMO, considering that these planes take time and rescourses to make there's not much sense in deviating into other theatres or time frames of the war. It makes much more sense, for now, to concentrate the efforts to the latter half of the war in the western front. That way the planeset gets fleshed out faster for special events and makes the default WWI arenas more uniform. So in that light my choises would be:
Albatros D.VaHalberstadt CL.II or CL.IV (There are many other German two-seaters to pick from, too)
SE5aSPAD XIII