WW1 did precipitate the development of two small arms that were ahead of their time, IMO, and used throughout WW2 and beyond:
The Browning Automatic Rifle (BAR) and the Thompson Submachinegun ("Tommy Gun").
I have to remember you that this is wrong for both weapons. Neither one was ahead of its time...:shocknaz:
The Thompson SMG was still on the drafting table, when the Germans already used the MP18 (Bergmann) as “trench broom” on the Western Front.
The BAR actually finds its place at the best in the genealogy of cheesy weapon design. If one considers it as a (poor) light MG, it came 15 years too late to be a forerunner since the Danish Madsen MG was successful employed already during the Russo-Japanese War.
Regarding the BAR as an assault rifle misses the point too, as it doesn’t use an intermediate round what makes the BAR much too heavy and shaky to be fired from the hip. At the end of the day the BAR was just some kind of overweight semi-automatic rifle just like the FG42...
Back to the initial question: Already at the end of WW1 the Germans had developed a wide variety of infantry weapons (SMG, hand grenade, flamethrower, infantry gun, lMG and hMG, grenade launcher) and tactics (infiltration) that helped to solve the deadlock of trench warfare, but came too late to finish the war before the arrival of the Americans. In fact there were absent in the arsenal of small arms only semi-automatic rifle and the assault rifle. Nevertheless the German did not need such weapons to smash the British 5th army during 1918 spring offensive.
What they would have needed rather than another small arms weapon, would have been food!, trucks, armored cars to exploit the initial success. Means they possessed sufficiently in 1940 what provided final success…
I strongly recommend on this subject the classic book from Bruce Gudmundsson: Stormtroop Tactics: Innovation in the German Army, 1914-1918.