Chemical weapons were developed and used with the hope that they'd end the war quickly, save lives, etc. The Germans used chlorine at Ypres in 1915 with this hope, but it did not pan out: aside from developing countermeasures (like gas masks), all sides began developing even nastier chemical compounds (e.g. mustard gas, phosgene, lewisite). So instead of ending the war it just made it more miserable and deadly.

I think searching for other "miracle technologies" somewhat misses the point about World War I: though technology played a major role in the conduct of the war (the entrenched nature of the war was largely due to technological changes), technology did not really play a big role in ending the war.

And the idea that a miracle technology can end a war has a very poor track-record by itself. Even in World War II it is not entirely clear that the atomic bomb deserves the credit for the end of the war, or that it would have ended the war had the Soviets not intervened simultaneously, etc. Even those who credit the bomb with ending the war acknowledge that it took the military defeat of Japan by conventional means to set up the preconditions in which the bomb could be effective. It is strange today how many people still share the view that "high technology can end wars" despite there only maybe being one example of this, and that there are many counterexamples (the Vietnam War, for example, was largely interpreted as a rebuttal to the idea that high tech assures military victory).

Separately, it is an interesting thing to note that several people involved in the chemical weapon programs of WWI later became important people in the various nuclear programs of WWII. The German chemist Otto Hahn, who was an assistant of Fritz Haber in developing chemical weapons, discovered nuclear fission and had a minor role in the German atomic program. Another German associate of Haber's, James Franck, emigrated to the USA and worked on nuclear reactors at the Chicago Metallurgical Laboratory. The American chemist James Conant, who had been in charge of the American effort to produce lewisite (the war ended before it could be used, so they dumped it off the coast of Baltimore), later became the President of Harvard University and was a major figure in the development of government defense science policy during WWII, and closely involved in the administrative side of the atomic bomb project.

The United States was that weapon. Had the war continued into 1919, the United States would have been capable of deploying many more times the number of men that Germany was able to following the Spring Offensive. As it was, the United States was never able to bring their full military capability to bear on the Western Front before the war ended in 1918 though they did make some valuable contributions to the Allied effort throughout the Hundred Days.

Technology wise, the Allies were continuing to progress their tank technology with the British developing tanks that served as armoured personnel carriers, the Mark IX as well as armoured gun carriers. These were first used in 1917 as mobile gun platforms but were better suited to supply carriers. Had the war continued, it is likely that we would have seen a greater wartime advancement of mechanised fighting vehicles and carriers.

Although not an invention, but more of a doctrinal approach regarding "achieving quick victory" was the fairly common idea, especially in the earlier parts of the war, was "the decisive battle". Many people thought of the war in the similar context to the Franco-Prussian war perhaps. In which a quick and decisive campaign is all that's needed to win a war. This was apparent on the western front, in the Italian front as well as in Gallipoli (hoping to kick the ottomans out with a swift attack on Constantinople). This was made unfeasible due to advances in defensive technology (such as machine guns) and the massively skyrocketed number of troops on all sides (this many million of troops on all warring sides had never occured before). A quick decivise batle was simply not possible. And as soon as the western front reached the Atlantic, the war bogged down do stagnatic trench warfare and generals eventually discovered, that indeed a quick vicotry is impossible.

Now the new question was how to break the trench-stalemate. And while the Germans made considerable advances with well-targeted artillery barrages, with a creeping barrage and shock troops penetrating deep into enemy lines, it eventually proved unsuccesful (Germans were stopped on the summer of 1918), although this might have been more to do with the limited manpower and economic capacity of the Germans, rather than doctrine itself.

Western allies, especially UK, but also France, started developing tanks, which over the course of the war, became more and more capable. Tanks were intented to be able to cross trenched, under infantry fire and penetrate deep into enemy lines, allowing a major break through. Germans also developed their tanks, but due to economic constraints, as well as doctrinal difference, their tanks didn't end up doing much before the war ended. Had the war extended to 1919, I'm sure tanks would've become much more important.

But directly to your question, I think that tanks (and lesser extent airplanes and other new weaponry) was the "miracle weapon" to win the war.

Now that I think of it, perhaps the German submarine strategy could also be added into the mix. A new technology that would starve the enemy in an unconventional way (although eventually proving insufficient).