There are tons of weapons other than rifle calibre machine guns:
-Le prieur rockets: used (widely) by the Entente against balloons. Erratic, so they are quite funny…
-Incendiary ammo: great against them, but if overheated it can set ablaze your own aeroplane.
-A scaled up Vicker to be used with 11mm incendiary ammo against drachens (the bigger the bullet, the bigger incendiary load).
-The Spad XII, a modified Spad VII with a 37 mm gun firing a kind of grapeshot through the hub.300 were made, I want one for me!
Of course, other weapons existed, but I don't think they will add somethin to RoF:
-Flechettes: darts paked to throw over the enemy trenches in 100's.
-Hand held guns: Because in 1915 you had to carry something to threaten them aboard: Colts, Mauser C-96 and such.
-Bricks, boots, a grenade if you were lucky, etc. Same as above, but against ground.
-Anything unorthodox that Alexander Kozakov tried: Chains, hooks, explosives…

Apart from this, the use of aircraft carried 37mm guns (specially by pusher two-seaters)against ground targets was conmon.There are tons of weapons explained in "flying guns of world war one".
Of course, give us first the chance to use the Lewis and Parabellum on two seaters, please.
